COMMISSION 27 OF THE I. A. U.
                   INFORMATION BULLETIN ON VARIABLE STARS
                                Number 2480

                                                       Konkoly Observatory
                                                       Budapest
                                                       17 February 1984

                                                       HU ISSN 0374-0676



         THE PHASE BEHAVIOUR OF THE GENEVA Z AND Delta(V1-G) PARAMETERS
                       FOR 41 Tau, 56 Tau AND 49 Cnc


  In the Geneva seven-colour system there are two parameters distinguishing
magnetic from nonmagnetic stars:

Delta(V1-G) defined by Steiger (1974)


         Delta(V1-G) = (V1-G) - 0.289(B2-G) - 0.302m

and reddening-free parameter proposed by Cramer and Maeder (1979, 1980)


    Z = 0.0255U - 0.1740B1 + 0.4696B2 - 1.1205V1 + 0.7994G - 0.4572m


which, however, is not valid for spectral types later than A3. They both, like
Ap-sensitive parameters in other photometric systems, measure the depth of a
broad absorption structure centred about 5200 A. Its origin is not yet clear
and this is the first reason why the phase behaviour of Z and Delta(V1-G) is
important for understanding of the Ap phenomenon. The other one is that
Delta(V1-G) and Z are fairly well correlated with the mean surface magnetic field
H_s if one considers the hotter Ap stars with H_s not exceeding 5 kGauss.
  So far the phase dependence of the strength of 5200 A structure has been
investigated for several stars (cf. e.g. Buchholz and Maitzen 1979, Gertner,
Muciek and Mikolajewski 1983, Pyper and Adelman 1983, Maitzen and Vogt 1983).
  Here we present the plots of Z and Delta(V1-G) versus phase for other three
stars, In each case a sinusoid is fitted by least-squares method. The errors
have been calculated for each point separately according to the definitions
of Rufener (1981) and vary from 0.004m to 0.01m. The clearly erroneous points
(marked by crosses) are omitted.

41 Tauri = HD 25823       (Figure 1)

  The phases are computed with ephemeris determined by Abt and Snowden
(1973):


     T(periastron passage) = 2421944.74 + 7.227424d E




The best fits are:

          Z = -0.0348m - 0.0092m cos(2pi phi + 1.57)

and

       Delta(V1-G) = 0.0262m + 0.0078m cos(2pi phi + 1.61)

  This star was also studied by Adelman (1983) who did not see any variability 
of the Delta a index which is also connected with the strength of the
5200 A structure.


                            [FIGURE 1]

 Figure 1   The single-wave fits to Z and Delta(V1-G) for 41 Tau. The
             observations marked by crosses are excluded



56 Tauri = HD 27309       (Figure 2)
  The ephemeris used is that of Musielok et al. (1980):

         JD(U_max ) = 2442299.51 + 1.56896d E

The following curves are obtained:


            Z = -0.0632m - 0.0106m cos(2pi phi - 0.44)

and
        Delta(V1-G) = 0.0478m + 0.0078m cos(2pi phi - 0.41)



                           [FIGURE 2]

 Figure 2    The single-wave fits to Z and Delta(V1-G) for 56 Tau. The
             observations marked by crosses are excluded

                           [FIGURE 3]

 Figure 3    The phase dependence of Z and Delta(V1-G) for 49 Cnc.
 




49 Cancri = HD 74521     (Figure 3)

 For phase computations the elements of Winzer (1974) are used:

          JD(U_max) = 2441616.50 + 4.2359d E


Both parameters seem to be invariable for this star so we did not make any
fitting. However, Adelman and Pyper (1979) suggest a possible variability of
the Delta a index.





              M. MUCIEK and J. GERTNER

            Instytut Astronomii
            Uniwersytet Mikolaja Kopernika
            87-100 Torun
            Poland

                   P. NORTH

              Observatoire de Geneve et
              Institut d'Astronomie
              de l'Universite de Lausanne
              1290 Chavannes-des-Bois
              Switzerland


                  F.G. RUFENER

               Observatoire de Geneve
               1290 Sauverny
               Switzerland







References:
Abt H.A., Snowden M.S. 1973 Astrophys.J.Suppl. 25, 137 [BIBCODE 1973ApJS...25..137A ]
Adelman S.J., 1983 Astr.Astrophys.Suppl. 51, 511 [BIBCODE 1983A&AS...51..511A ]
Adelman S.J., Pyper D.M., 1979 Astron.J. 84, 1726 [BIBCODE 1979AJ.....84.1726A ]
Buchholz M., Maitzen H.M., 1979 Astr.Astrophys., 73, 222 [BIBCODE 1979A&A....73..222B ]
Cramer N., Maeder, A., 1979 Astr.Astrophys. 78, 305 [BIBCODE 1979A&A....78..305C ]
Cramer N., Maeder A., 1980 Astr.Astrophys. 88, 135 [BIBCODE 1980A&A....88..135C ]
Gertner J., Muciek M., Mikolajewski M., 1983 I.B.V.S.  No.   2268
Maitzen H.M., Vogt N., 1983 Astr.Astrophys. 123, 48 [BIBCODE 1983A&A...123...48M ]
Musielok B., Lange D., Schoneich W., Hildebrandt G.,  Zelwanowa,  E.,
   Hempelmann A., 1980 Astr.Nachr. 301, 71 [BIBCODE 1980AN....301...71M ]
Pyper D.M., Adelman S.J., 1983 Astr.Astrophys.Suppl. 51, 365 [BIBCODE 1983A&AS...51..365P ]
Rufener F., 1981 Astr.Astrophys.Suppl. 45, 201 [BIBCODE 1981A&AS...45..207R ]
Steiger P., 1974 Publ.Obs.Geneva Ser. A., 81 [BIBCODE 1975PGenA..81...53S ]
Winzer J.E., 1974 Ph.D. Thesis, University of Toronto [BIBCODE 1974PhDT........61W ]