COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                       Number 1142


                                Konkoly Observatory
                                Budapest
                                1976 June 15




           THE PERIOD OF THE CEPHEID CV MONOCEROTIS 




      During 10 nights in the interval 27 February to 10 March 1976 the Cepheid
variable CV Mon was observed photoelectrically with the University of Toronto 61-cm
telescope on Cerro Las Campanas. On the night of February 28/29 the variable was
observed to go through a maximum, reaching V = 9.90 and B = 11.06. The estimated
time of maximum light, JD 2442837.605 +- 0.025, occurred 9 hours earlier than the time
expected according to the elements calculated by Wachmann (1964, Astron.Abhand.
Hamburger Sternw. 7, 203). New elements for this variable are clearly needed. However,
as we demonstrate here, the available observational data do not yield an unambiguous
solution for these.
      The newly-observed time of maximum listed above indicates that Wachmann's
period of 5.3788d should be reduced to 5.3786d. The other possible interpretation,
namely that the recently-observed maximum occurred almost a full cycle late, leads to
a period of 5.3808d. The uncertainty in both estimated periods is less than 0.0001d.
      The period of CV Mon has also been derived by fitting the new photoelectrically-
observed light curve (Turner, 1976, in preparation) to the observations
of Arp (1960, Astrophys. J. 131, 322). Care was taken to insure that both sets of
observations were on the same system. This analysis resulted in a period of 5.3790d +-
0.0001d s.e., which disagrees with both of the above estimates.
      The discrepancy may result from the comparison of photographic (Wachmann)
and photoelectric (Turner, Arp) observations. However, it is also conceivable that
a period change has occurred for this variable. Further photoelectric monitoring
of CV Mon would clearly be of value for resolving this problem.




                        D.G. TURNER

                        David Dunlap Observatory
                        University of Toronto
                        Richmond Hill, Ontario
                        Canada