COMMISSIONS 27 AND 42 OF THE IAU 
         INFORMATION BULLETIN ON VARIABLE STARS 
                      Number 3724 
 
                                         Konkoly Observatory 
                                         Budapest 
                                         7 May 1992 
 
                                         HU ISSN 0374 - 0676 
 
 
   Updated ephemerides for the cataclysmic variable EX Hydrae 
 
Cyclical variations in the orbital periods of close binary systems may be 
caused by solar-type magnetic cycles of the component stars (Warner 
1988) although the mechanism is still under discussion (see Applegate 
1992 and references therein). The cataclysmic variable EX Hya shows 
orbital period fluctuations consistent with a ~ 20 year cyclical variation 
(Jablonski & Busko 1985; Bond & Freeth 1988), however since observations 
cover only ~ 1 of these cycles continued monitoring is required. 
We observed EX Hya on several nights over the interval 1991 Jan 10-22, 
five years after the last published data. We used the 0.75-m telescope 
of the South African Astronomical Observatory, making 2-s integrations 
with the UCT photometer and an unfiltered RCA Ga As tube. 
  The 98-min orbital period in EX Hya is manifest as sharp, shallow 
eclipses which make excellent fiducial marks. We timed 13 eclipses, 
measuring their midpoints by fitting with a Gaussian profile (Table 1). 
To investigate the long term behaviour we have calculated the residuals 
about the linear ephemeris of Mumford (1967), corrected to barycentric 
dynamical time: 
 
        TDB_eclipse = 2437699.94179 + 0.068233846 E 
 
These residuals are plotted in Fig.1 along with previously published data 
(from Bond & Freeth 1988 and references therein). The new timings are 
consistent with the suggested cyclical variation; a sinusoidal fit gives: 
 
     O - C = - 0.00027 + 0.00028 sin[360^d(E/93500)  + 7deg] 
                  +/-3      +/-4           +/-6500 +/-24
 
where the period of 93500 cycles corresponds to 17.5 +/- 1.2 years. 
 
 
               Table 1: Timings (TDB-2448200) 
 
Eclipses   72.50212   76.59601    Spin       72.5123   75.5416 
66.56632   72.57081   78.57531    67.5824    72.5635   75.5894 
67.58930   73.59361               68.5629    72.6085   76.5730 
68.54457   74.54889               68.6100    73.5891   76.6081 
68.61324   74.61851               69.5430    74.5687   78.5623 
69.56834   75.57312               69.5807    74.6096   78.6098 


                           [FIGURE 1]

              FIGURE 1: Residuals of eclipse timings

 
  EX Hya contains a magnetic white dwarf which produces a prominent 
sinusoidal photometric modulation at the 67-min rotation period. 
This period is decreasing secularly, presumably due to the accretion of 
angular momentum. We have measured the times of spin maxima in the 
light curves by eye, for consistency with previous work (Table 1). The 
residuals against the linear ephemeris of Vogt et al. (1980) are shown in 
Fig. 2 along with the compilation of previous data. A quadratic fit to 
the data gives the ephemeris: 
 
TDB_67-max = 2437699.8914(5) + 0.046546504(9)E - 7.9(4) x 10^-13 E^2 


                            [FIGURE 2]

           FIGURE 2: Residuals of spin maximum timings

 
We thank the SAAO for the allocation of telescope time. 
 
 
C. HELLIER and L. N. SPROATS, Mullard Space Science Laboratory, 
Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK. 
 
 
References: 
Applegate, J. H. 1992. Astrophys. J., 385, 621.  [BIBCODE 1992ApJ...385..621A ]
Bond, I. A. & Freeth, R. V., 1988. Mon. Not. R. astr. Soc., 232, 753.  [BIBCODE 1988MNRAS.232..753B ]
Jablonski, F. & Busko, I. C., 1985. Mon. Not. R. astr. Soc., 214, 219.  [BIBCODE 1985MNRAS.214..219J ]
Mumford, G.S. 1967 Astrophys. J. Suppl., 15, 1.  [BIBCODE 1967ApJS...15....1M ]
Vogt, N., Krzeminski, W. & Sterken, C., 1980. Astr. Astrophys., 85, 106.  [BIBCODE 1980A&A....85..106V ]
Warner, B., 1988. Nature, 336, 129.  [BIBCODE 1988Natur.336..129W ]