COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1809

                                    Konkoly Observatory
                                    Budapest
                                    1980 June 23




        ONE MORE CRITERION FOR DISTINGUISHING
  BETWEEN W VIRGINIS STARS (CW) AND CLASSICAL CEPHEIDS (C_delta)





    As is known the study of morphological peculiarities and
space-kinematical characteristics of cepheids cannot always
lead to a reliable distinguishing between the CW and C_delta stars,
due to which about 1/3 of cepheids from GCVS (1969) have no
corresponding designation.
    We proposed a criterion of cepheids distinguishing according 
to the maximum width of the loop in the U-B, B-V plane,
namely L_U-B. The dependence of the L_U-B from log P was investigated 
for all cepheids with available U,B,V photometry. Fig. 1
shows that the CW loops are wider than the Cdelta ones of corresponding 
periods. However, the loop width of some reliable CW,
such as UY Eri, V553 Cen, kappa Pav, AP Her and AL Vir is comparable
with that of C_delta.
 
                        [FIGURE 1]

   Fig.1. The loop width L_U-B in the U-B, B-V diagrams vs.
 logarithm of the period. The filled and open circles show the
 Cepheids of types CW and Cdelta, respectively. The signes marked with
 crosses mean the cepheids without hydrogen emission lines in
 their spectra, ones placed in circles and rhombes - the
 presence of weak and strong hydrogen emission, respectively.

   According to Wallerstein (1958) and Lloyd Evans et al.(1972)
there is no hydrogen emission of these stars (kappa Pav lacks for
information of hydrogen emission). In the same paper Wallerstein
found that the hydrogen emission seems to have a marked maximum
among stars of period 20-30 days. Among CW stars of short period
a weak hydrogen emission was found by Abt and Hardie (1960) for
BL Her. We know of the only classical cepheid which was found to
have very weak hydrogen emission (Kraft, 1956), X Cyg. All the
other C_delta stars have only metall emission lines in their spectra
and no hydrogen emission.
   As Fig.1 shows, there is a correlation between the loop
width L_U-B and the strength of the hydrogen emission lines.
Hydrogen emission and the ultraviolet excess are likely to display 
one and the same phenomenon connected with the shock wave
propagation in the atmosphere of these stars. The effect of the
shock waves on the colour indices is considered by Klimishin
(1972). Our calculations of the shock wave front emission provide
the value of the ultraviolet excess, which agrees in the order
with the observed L_U-B (Batyushkova, in press).
   The dependence L_U-B on log P shown in Fig.1 may be used as a
criterion of distinguishing between CW and C_delta variables in addition 
to the existing criteria. However, our criterion is not simple.
The presence of the ultraviolet excess, which exceeds 0m05 and
0m08 for the stars of P<10d and P>10d, respectively, is the cause
for classifying them as CW. The lack of such excess in a cepheid
variable cannot be the evidence of its adherence to Cdelta class.








                 B.N. BATYUSHKOVA, G.E. ERLEKSOVA

               Astrophysical Institute of Tadjik
               Academy of Sciences, Dushanbe, USSR
References:
Abt, H., and Hardie, R., 1960, Ap.J. 131, 155 [BIBCODE 1960ApJ...131..155A ]
Batyushkova, B., in press, Tadjik.bull.
Klimishin, I.A.,1972,in "The shock waves in nonuniform media"
      the Lvov University
Kraft, R.P., 1956, P.A.S.P., 68, 137 [BIBCODE 1956PASP...68..137K ]
Kraft, R.P., 1960, in"Stellar atmospheres"ed.J.L.Greenstein
Lloyd Evans, T.,Wisse, N., and Wisse, M.,1972,M.N.R.A.S. 159, 67 [BIBCODE 1972MNRAS.159...67L ]
Wallerstein, G., 1958, Ap.J. 127, 583 [BIBCODE 1958ApJ...127..583W ]