COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                     NUMBER 366

                                 Konkoly Observatory
                                 Budapest
                                 1969 July 22


    EVIDENCE FOR VERY STRONG LIGHT-CURVE VARIATION
              OF THE CEPHEID IU Cyg

      Wachmann (A.A. 1966, Abh. Hamburg 6, 283) has derived
the light-curve and light-elements of the cepheid IU Cyg
for the time interval 1907/1960 from 424 photographic 
observations, tied into a photoelectric sequence. He found
two instantaneous systems of elements, the later one being:

      J.D. (max) = 243 6788.7 + 31.31451d n,

      Eggen's (O.J. 1969, Ap. J. 156, 617) photoelectric 
observations of IU Cyg, made between June, 1962, and October,
1963, agree near maximum light very well with these element.
They indicate, however, that the light curve of the variable
has undergone drastic changes: Eggen's descending branch is
much steeper and the ascending branch accordingly flatter;
although Eggen's minimum is poorly determined, it is
apparently shifted by about 0.4 of the period to earlier
phases. While on the basis of the older light curve the
star was classified by Petit (M. 1960, Ann. Astr. 23, 710) as a
Delta Cep-type variable, Eggen's light curve is typical of
W Vir-stars, which conforms with the period changes found
by Wachmann and the great value of z, which one obtains
from a population I period-luminosity relation (Fernie, J. D.
and Hube, J. O. 1968, A.J. 73, 492).
      It is quite unlikely that the change of the light-
curve is only the effect of photographic errors; this
possibility is practically excluded by the shift of the minimum.
A realistic estimate of the photographic errors even 
indicates that the increase in amplitude Delta m_B from 1.3m to >=1.6m
is real.
      Additional evidence for the supposed light-curve
changes comes from the 8 photoelectric observations by
Mitchell et al. (1960, Tonantzintla Bol. 3, 153), which cover
the interval from September, 1960, to December, 1961. The
four observations near maximum fit Eggen's light-curve
reasonably well; the four observations near minimum, 
however, exhibit a quite erratic behaviour. It seems indicated,
therefore, that IU Cyg went through an unstable transitional
phase in 1960/61 before assuming the light-curve defined by
Eggen's 1962/63-observations.

July 10, 1969



         REMARKS ON THE LIGHT ELEMENTS OF CS Mon

      The period of the Delta Cep-variable CS Mon was 
determined by Ahnert et al. (Ver. Sonneberg 1, 47). On the basis
of only 70 visual observations they found a period
6.68073 days. Adopting this period Eggen (1969, Ap. J. 156, 617
found from eight photoelectric observations a most unusual
light curve with a narrow, bright secondary maximum, which
he considered to be confirmed by four photoelectric observations
made by Walraven et al. (1968, B.A.N. 14, 81) and
transformed into the UBV-system by Mitchell et al. (1964,
Tonantzintla Bol. 3, 153). However this confirmation holds
only if Walraven's observations are shifted by an arbitrary
amount of about 0.55 periods. In addition the maxima 
determined by Ahnert et al. (Ver. Sonneberg 1, 47) do not agree
in phase with Eggen's light curve.

			[FIGURE 1]


      This evidence strongly suggests that the period for
CS Mon is incorrect. The available data do not suffice to
determine a reliable period; however, they do suggest that
the period lies near four days. Eggen's observations can
well be represented by an asymmetric, but not unusual light
curve with the following light-elements (see illustration,
dots):
      J.D.(max) = 2437899.0 + 4.16915 n 

The observations by Walraven et al. (open circles) scatter
around the light curve by +/- 0.14m, which does not seem to be
prohibitive considering the necessary magnitude transformation.

July 15, 1967.


                      G. A. TAMMANN
          Astronomisches Institut der Universitat Basel