1. Preface to the 6th Edition

Seven and a half years after the publication of the previous (5th) edition of the Catalogue of Cataclysmic Binaries, Low-Mass X-Ray Binaries and Related Objects (Ritter 1990, hereafter R90), and almost four years after the deadline of an updated list of cataclysmic binaries published in 1995 (Ritter & Kolb 1995, hereafter RK95), the amount of new literature and the number of new objects to be included have again grown so much that it seems worthwhile to publish an updated edition. The philosophy and the purpose of this catalogue (now in its 6th edition) have been outlined in the preface to the 3rd edition (Ritter 1984, hereafter R84) and will not be repeated here. Rather let us briefly recall some of the developments which, over the past seven and a half years, have had (and still have) a major impact on this catalogue:

• First, we have seen the successful launch and operation of a number of satellites, e.g. of Rosat, Ginga, ASCA, EUVE, GRO, and XTE, which have led to the detection of many new sources or even new classes of compact binaries (e.g. AM Her stars and supersoft X-ray sources by Rosat or EUVE, X-ray transients (Black Hole candidates) by Ginga and GRO), or to new insights concerning previously known sources (e.g. the detection of kHz X-ray oscillations in low-mass X-ray binaries by XTE observations).
• Second, we have seen the publication of A Catalog and Atlas of Cataclysmic Variables by Downes & Shara (1993), and most recently of its updated 2nd edition by Downes et al. (1997), both of which are now the primary sources for accurate coordinates and finding charts for cataclysmic variables.
• Third, since the publication of the 5th edition of this catalogue four new issues of the Name List of Variable Stars have appeared (the 70th by Kazarovets & Samus (1990), the 71st by Kazarovets, Samus & Goranskij (1993), the 72nd by Kazarovets & Samus (1995), and finally the 73rd by Kazarovets & Samus (1997)). These lists provide the definitive variable names which are also used as primary names in this catalogue. We note in passing that the catalogue by Downes et al. (1997) does not yet include the designations provided by the 73rd Name List.
• Fourth, we note the increasing contribution of amateur astronomers in the field of cataclysmic variables. Equipped with CCD photometers, amateur astronomers are now in a position to do photometry of moderate time resolution of rather faint objects, even with telescopes of rather small apertures. Such photometry is for example adequate to track down the superhump periods of presumed SU UMa-type stars, or to determine the orbital periods of eclipsing binaries. Results of these amateur activities, which can be found on the Web page of the Variable Star Network (VSNET) at http://www.kusastro.kyoto-u.ac.jp/vsnet, have also been incorporated in this compilation.

Compared with the 5th edition, the number of objects listed has almost doubled. The current version of this catalogue provides tabulated data and references for 414 objects (318 cataclysmic binaries, 47 low-mass X-ray binaries and 49 related objects).

The fact that on the one hand the number of objects, the number of references per object and the length of the Who's Who? section are steadily increasing, and that on the other hand the total length of the catalogue must remain within acceptable limits, makes it necessary to drop some of the information given in previous editions. This is done by suppressing essentially all those references already given in RK95 (for the cataclysmic variables), or in the 5th edition (R90) (for the other objects). Since the previous three editions (R84, Ritter 1987, hereafter R87, and R90) are published in an easily accessible journal, all the information is still available. Earlier references are only repeated if they are required for cross-reference (see below). Accordingly the 6th edition provides:

• the tables for all three object classes (cataclysmic binaries, low-mass X-ray binaries and related objects) in full;
• for each catalogued object a selection of references to the literature, published either after 1 September 1993 (the deadline of RK95) for cataclysmic binaries, or after 31 December 1989 (the deadline of R90) for the other objects. Earlier references are only included if they are needed for cross-reference, or in cases where there have been few or no new publications of relevance;
• a list of selected references to published finding charts. Additional references may be found in previous editions;
• the Who's Who? file, a cross-reference list of alias names of the objects catalogued.

Thus the catalogue is complete and self-contained in the tables and in giving cross-references to alternative object designations. With respect to the bibliography given in the reference sections and to the references to published finding charts it is essentially a supplement to RK95 in the case of cataclysmic binaries, and to R90 in the case of the other objects.

We should also like to mention three major changes which we are introducing with this edition, and which, so we hope, will make the catalogue even more valuable:

• First, for better readability we now use upper and lower case characters instead of upper case only as in previous editions.
• Second, in order to save space, we have changed the style of referencing. We use shorter abbreviations for the major journals (e.g. ApJ instead of Astrophys. J.) and in case of references with more than five authors we quote only the name of the lead author, followed by "et al.'' and the number of authors in parenthesis.
• Third, as we have already done in RK95, we provide now limited information about where the values given in the tables are taken from. How this is done is explained in the Introduction.

Every effort has been made to avoid errors and to keep the lists up to date. Nevertheless, the authors are well aware of the fact that also this edition will contain errors and may be incomplete with regard to the criteria stated in the preface to the 3rd edition (R84). It is certainly incomplete with respect to the references quoted. However, it should be stressed that no attempt has been made to provide a complete bibliography. Rather, the aim is to give a selection of references that should allow the user to find his way through the literature addressing mainly the binary properties of the objects in question.

All the tabular material contained in this catalogue is published in electronic form only. It is available from the CDS via anonymous ftp to cdsarc.u-strasbg.fr
(130.79.128.5), via http://cdsweb.u-strasbg.fr/Abstract.
html, or from the MPA Web page at http://www.mpa-garching.mpg.de/Binary/ukolb/ed6.html. In addition to the machine-readable version, the ps-files for a printable version are also available at both Web sites.

For the current version of this catalogue, literature published before 30 June 1997 has, as far as possible, been taken into account.