COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                    Number 1658

                                    Konkoly Observatory
                                    Budapest
                                    1979 August 8




NEW RESEARCH FOR PERIODS OF Ap STARS OBSERVED AT THE ESO - III



    A new series of six Ap stars has been observed in the uvby
system with the photometer attached to the ESO 50-cm telescope.
However the measurements are somewhat less accurate than for the
previous series (see IBVS Nos. 1280, 1391, 1451) because less time
has been devoted to the new stars in order to reobserve Ap stars
previously measured. These new measurements have been made from
the end of November to December 20, 1978. They have still been
analysed with the same method (P. Renson, Astron. and Astrophys.
63, 125, 1978) to find periods. This gives the following results.
 
    star          sp.type      period             range(mag.)
 --------------------------------------------------------------------
 HD 24155=HR 1194 B9pSi        2.53d+-0.03d  from 0.06 in y to 0.1 in u
 HD 27376=nu^4Eri B9pMn        0.51d ?           very small
 HD 32549=11 Ori  B9pSi        4.63d+-0.09d  from 0.03 in y to 0.1 in u
 HD 39317=137 Tau B9pSiEuCr    2.63d ?       near 0, except in u (0.05)
 HD 42536=HR 2195 A0pSrCr      (3.6d ?)      small,mainly in y and b
 HD 42657=HR 2202 B9pHgMn      0.724+-0.008d from 0.02 in y to 0.04 in u
 ---------------------------------------------------------------------

    Due to the lack of precision (one measurement for each comparison 
star instead of two, and two for the Ap star instead of three,
for many individual values of Ap-(C_1+C_2)/2) and the smallness of
some ranges, several results are very uncertain. The periods given
with a question-mark, especially for HR 2195, are probably spurious
periods, these stars may have periods which are too long to show
significant variations during our observations.
    It is noteworthy that HR 2202, which is a Hg-Mn star, is
clearly periodic; however the range of the variation is small.
We think that the Hg-Mn stars may be periodic variables such as
the other Ap stars, but that they are believed to exhibit no
periodic variations, only because the variation range is generally
very small.

                       P. RENSON and J. MANFROID
                       Institut d'Astronomique de
                       l'Universite de Liege
                       5, avenue de Cointe
                       B-4200 Cointe-Ougree (Belgium)