The Washington Visual Double Star Catalog, 1996.0 (WDS) is the succes- sor to the catalog of the same name dated 1984. Three earlier double star cata- logs in this century, those by Burnham (1906, denoted the BDS), Innes (1927, denoted the SDS), and Aitken (1932, denoted the ADS), each covered only a por- tion of the sky. In 1963 Lick Observatory published the "Index Catalog of Visual Double Stars, 1961" (Jeffers and van den Bos), denoted the IDS, and in 1984 we in turn produced the WDS. Both the IDS and the WDS cover the entire sky, and the WDS is intended to contain all known visual double stars for which at least one differential measure has been published through the end of 1995. The reader is reminded that the WDS is therefore not inclusive of all pairs which have been more casually noted, although it does, in fact, include some of them.

The data base upon which the present WDS rests consists of the approx- imately 180000 measures transferred from the Lick Observatory in 1965, which, with few exceptions, covered the interval 1927-1963, augmented by all subse- quent measures (about 118000), and some 154000 pre-1927 means. These latter are the result of a continuing project to build a truly comprehensive and complete data base. Thus, approximately 452000 individual means, representing over a million individual observations, form the basic material from which the WDS is constructed. Also, we have not included a small amount of data because of unre- solved questions concerning it.

The WDS contains 78100 double stars, counting, as did the IDS, multi- ple components in the same system as separate pairs. Comparison of the present catalog with its IDS predecessor will reveal that a considerable number (about 1000) of previously catalogued objects have been removed from the WDS in the process of editing the data. The most common reason for removal is that the ob- ject is not certainly double. All such objects are being retained in a file en- titled "Stars Suspected of Duplicity" pending solution of the individual prob- lems.

Because 1900 coordinates are increasingly obsolete, the present cata- log is ordered by the 2000 coordinates. While such coordinates may be used as a primary identifier, the fact is that many pairs have somewhat uncertain po- sitions. In fact, the only unique identifier is the discoverer's designation and number. Unfortunately, many discoverers have failed to assign numbers to their pairs. We have remedied this problem by assigning sequential numbers to such pairs in the case of deceased observers, and we urge current observers to provide numbers. We also wish to emphasize that the WDS is not intended to be an "astrometric" catalog in the sense usually employed, but rather is to be con- sidered a finding list for observers, as well as serving as a notification of which objects are known doubles or multiples, along with some basic astrometric and astrophysical parameters. In addition, the WDS offers information on the amount of motion and frequency with which a given object has been observed, which is sufficient, in fact, to provide a basis for the construction of obser- vational programs. Finally, the data can be used (with proper precautions) to analyze certain statistical properties of visual double stars. Of course, as has been true for many years, the individual measures of specific objects are available and will be supplied to requestors provided that the amount of data desired is reasonable.

Historically, the collection of data on cards in order to construct the IDS required certain compromises due to the limitations of the 80-column format, and this has been propagated into the WDS. In the WDS 1996 version, con- siderable reformatting has allowed inclusion of two-decimal V magnitudes when available, as well as MK spectral types. These data have been obtained from many sources, most important of which have been the Bright Star Catalogue, the Michigan Spectral Survey, and the SIMBAD data base. In the latter case, all of the Durchmusterung stars have been individually compared with SIMBAD data. In the case of magnitudes, the most commonly found datum was the combined V magni- tude. In such instances the observational data base was examined in order to define a more reliable "mean" magnitude difference. This was then used, together with the total magnitude, to determine the individual values. However, it re- mains true that the magnitudes presented in this version of WDS are the most in- homogeneous parameters, and we urge users interested in statistical studies to allow for this fact. We are also encouraged to realize that projects now under way will do much to remedy this situation in the foreseeable future.

2. References.

While both Burnham and Aitken provided fairly complete references to the material included in their double star catalogs, Jeffers and van den Bos chose to omit such data. We believe this was a mistake, and consequently pro- vide such a list. We wish to remind the reader, however, that this list, large as it is, by no means represents all references to visual double star observa- tions. In fact, it is a list which indicates that all, or in some cases only part, of each individual list is included in the present data base. Naturally, we hope that the reference list will become truly comprehensive when we have completed addition of all of the old data! The reader will also note that ap proximately 1% of the references are incomplete, a fact of which we are well aware. The reason for this is that the many hundreds of references supplied us by Lick were often very incomplete, and, for example, almost never included the dates of publication. Moreover, there exist in the data base reference codes for which no references were supplied (fortunately not very many). Many months of library research have been expended in bringing the reference list to its present state, but more work is still needed.

3. Description of Data in the WDS.

Right Ascension and Declination for 2000.

The hours, minutes, and tenths of minutes of Right Ascen sion for 2000, followed by the degrees and minutes of Declination for 2000, with + and - indicating north and south declinations. The positions given represent our best estimates of these values. Where possible, these are based on the ACRS and PPM data, with proper motion incorporated.

Discoverer.

The discoverer, identified by his one-to-three letter code, and his discoverer's number, if assigned. The reference list should be consulted first to identify individuals. However, some discoverer designations do not appear in this list, for various reasons, and a supplementary list is provided imme diately following the references.

Components.

Components, when the object has more than two. The Lick IDS scheme has been discontinued, and components are now referred to by the traditionally employed lower-case letters. The rather awkward upper-case designations, e.g. ABXC, have been changed to the form AB-C, etc. We have noted some confusion on the part of observers and students alike, as to how to designate components in multi- ple systems. Traditionally, these have been designated in order of separation, thus AB, AC,...., or in the cases where close pairs are observed blended, AB-C, AB-D,.... In some instances, differing resolution limits produce situations where observations are intermixed, thus AC, AB-C, and so forth. (In all too many cases, carelessness on the part of the observer does not permit us to determine with certainty how the observation is to be interpreted). There are also many instances where later observations have revealed a closer companion; these are designated Aa, Bb, etc. In a few cases wider, later discoveries have also been so denoted.

Dates.

The dates of the first and last observations of an object (of satisfactory accuracy), up to the closing date of 1996.0.

Number of measures.

The number of measures of the object. When there are more than 99, the number 99 is used. For smaller numbers of ob- servations, the value in these columns is intended to accu- rately reflect the actual number.

Position angles.

Position angles in degrees for the dates listed in columns 24-29. For some pairs, the discoverers have given only crude estimates, such as NF or SP; in such cases these are listed. Position angles are unprecessed in this catalog (i.e. they are for the mean date of observation).

Separation.

The distances in seconds of arc, and tenths, for the dates listed in columns 24-29. Some separations are treated by the codes found in columns 79-80, or in the Notes.
In cases where there has been no appreciable motion observed over the duration of observation, only one position angle and separation is listed.

Magnitudes.

Magnitudes of the two components. See the Introduction for discussion.

Spectral type.

Spectral type of A, or of two components, if space permits. See the Introduction for discussion.

Proper motion.

The components of the proper motion in seconds of arc per 1000 years, in right ascension reduced to great circle, and in declination. Specifically, this catalog lists 15mu(alpha) cos(delta), where mu(alpha) is in seconds of time, and mu(delta) is in seconds of arc. Both quantities are reduced to a time unit of 1000 years. The exceptions are those ob- jects coded P, Q, and R, for which an explanation is given under the description for Columns 79 and 80. + and - indicate eastward and westward motions in Right Ascension, and + and - those north and south in Declination.

Durchmusterung.

Durchmusterung number of the object in the system used by the Henry Draper Catalogue: Bonn from +89 degrees to -22 degrees inclusive, Cordoba from -23 degrees to -51 degrees inclusive, Cape Photographic from -52 degrees to -89 de- grees inclusive. When a star is not contained in the Durch- musterung proper to its declination zone, but is contained in another Durchmusterung, this is so indicated in the Notes. Other compnents having different Durchmusterung num- bers are also indicated in the Notes.

Notes.

The following codes are contained in these columns:

N:

Notes found in the Notes table.

O:

Orbit. Where motion has exceeded 360 degrees, no values are listed in the position angle and sepa- ration columns. Orbits may be found in the "Fourth Catalog of Orbits of Visual Binary Stars" (Worley and Heintz, 1983), and in the Information Circulars published by Commission 26 of the IAU, as well as in the literature.

P:

100 year proper motion in right ascension.

Q:

100 year proper motion in declination.

R:

100 year proper motion in right ascension and decli- nation.

a:

Pair appears in an appendix list, not part of the discoverer's regular numbering system.

r:

Pair was listed, but in a "rejected" list, not part of the discoverer's regular numbering sequence.

s:

Pair has other discoverer's designations and num- bers in the literature due to duplicate discovery. We have given credit to the earliest discovery observation (and correct identification) reported in the literature. The List of Synonyms follows the List of Additional Discoverers.

p:

Coordinates and proper motions from the ACRS, PPM, IRS, and FK5 catalogs.

6:

Separation given in minutes of arc rather than se- conds.

4. Notes.

The original IDS Notes were not supplied to us in machine-readable form, but were made available by Dr. W. H. Warren, Jr. As conceived originally, these Notes concerned mainly identifications, and little general astronomical information was included. We have extensively revised the Notes to include much more information on orbital motion and multiplicity (including astrometric, photometric, and spectroscopic), variability, composite spectra, etc. In the case where an orbit exists for a pair, we give the period (P) and the semi-major axis (a) in seconds of arc, plus an indication of the direction of motion.

5. Errors , Future Plans.

We, like our predecessors, have found and corrected very many errors of all types in the compilation of the WDS and its underlying data base. More remain undetected, and we urge the users to inform us of these so that we may speedily correct them.

It is our intention to maintain the data base in a continuously- updated form, incorporating all corrections, and to strive to complete the addition of all of the older material. As mentioned above, we also hope to be able to improve the magnitude system to an appreciable extent. Consequently, updated tape/CD and on-line World Wide Web versions of the WDS should be achievable at much shorter intervals than has characterized the publication of "hard-cover" visual double star catalogs in this century. We believe this is a desirable evolution.

6. Acknowledgements.

We are first of all grateful to our predecessors who have, for more than a century, carefully and systematically collected and disseminated visual double star data, thereby laying the foundations for this work.

Many members of the U.S. Naval Observatory staff, including summer students, have participated in this project to a greater or lesser extent. Those who have made larger contributions include A. Klugh, M. Miranian, and V. Kallarakal. Our librarian, Mrs. B. Corbin, has been of great help. Many astron- omers who have used the data base have kindly sent us corrections. Finally, Dr. Wayne H. Warren, Jr., initially provided us with a machine-readable version of the IDS Notes, which has subsequently evolved into the present, expanded, WDS Notes. To all of these persons we express our appreciation and gratitude. We have also made extensive use of the invaluable SIMBAD data base maintained by the Astronomical Data Center of the Strasbourg Observatory.

7. References.

Aitken, R.G. 1932

New General Catalogue of Double Stars within 121 de- grees of the North Pole. Carnegie Institution of Washington.

Burnham, S.W. 1906

A General Catalogue of Double Stars within 121 degrees of the North Pole. Carnegie Institution of Washington.

Innes, R.T.A. 1927

Southern Double Star Catalogue, -19 to -90 degrees. Union Observatory, Johannesburg, South Africa.

Jeffers, H.M. & 1963

Index Catalogue of Visual Double Stars, 1961.0 Pub. van den Bos, W.H. of the Lick Observatory, XXI.

Worley, C.E. & 1983

Fourth Catalog of Orbits of Visual Binary Stars. Pub. Heintz, W.D. U.S. Naval Observatory XXIV, Pt. VII.