COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 2629


                                  Konkoly Observatory
                                  Budapest
                                  22 November 1984

                                  HU ISSN 0374 - 0676




          THE P-L-C RELATION OF GALACTIC FIELD
          CEPHEIDS AND THE P-L AND P-C RELATIONS
          OF OPEN CLUSTER AND ASSOCIATION CEPHEIDS


In our paper (Opolski and Ciurla, 1984) we have presented  the arguments
in favour of large value of the colour term coefficient in the P-L-C relation
for galactic field Cepheids. As the continuation of this investigation one
of us (T.C.) has worked out a modification of least squares and maximum
likelihood methods, which seems to be the most suitable for calculation the
P-L-C relation.In this new approach the log P is regarded as an accurate,
faultless parameter and M_<V>  and <B - V>_0  as burdened with accidental 
errors. The details of this method will be published in a separate paper.
Using the data for 52 galactic field Cepheids published by Opolski (1982)
(omitting 5 Cepheids which are suspected as overtone pulsators and 7 ones
belonging to open clusters and associations), we got by means of this new
method the P-L-C relation:

M_<V> = 5.40 <B - V>_o - 5.20 log P - 3.44                         (1)
      +-0.81           +-0.84       +-0.60
or, considering that
<B - V> = <B> - <V> + 0.028
we have
M_<M> = 5.40 (<B> - <V>)_0  - 5.20 log P - 3.29                    (2)

Here we have again the large value of the colour term coefficient, which
supports our previous results.
In order to test the position of the open cluster and association Cepheids
in relation to the plane determined by equation (2) in the three dimensional
space: M_<V>, log P, and <B> - <V> we have used the P-L and the P-C 
relations obtained for these stars by Fernie and McGonegal (1983):

M_<V> = -1.61 - 2.882 log P
        +0.10  +0.84                                          (3)
( <B> - <V> )_0 = 0.32 + 0.418 log P.
                 +0.04  +0.032

Geometrically the relations (3) can be treated as equations of a straight line
in the same space. It is easy to verify that between the line (3) and the
plane (2) there is a small angle amounting to only 9+- 3 arcminutes and the
line (3) intersects the plane (2) at the point: M_<V> =  -3.88, log P = 0.79
and (<B> - <V> )_0 = 0.65, that is close to the mean position of the 
investigated field Cepheids. Taking into account the accuracy of these data, we
may conclude that the line (3) is just placed on the plane
open cluster and association Cepheids satisfy also the formula (2), though
they do not determine any P-L-C relation, being placed along the line (3).




                         T. CIURLA and A. OPOLSKI
                         Wroclaw University Observatory
                         ul. Kopernika 11
                         51-622 Wroclaw, Poland





References:

Fernie, J.D. and McGonegal, R. , 1983, Ap.J., 275, 732. [BIBCODE 1983ApJ...275..732F ]
Opolski, A., 1982, Commun. Konkoly Obs. Hung. Acad. Sci. Budapest. No. 83, 227.
Opolski, A. and Ciurla, T., 1984, Inf. Bull. Var. Stars, No. 2528.