COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1663

                                     Konkoly Observatory
                                     Budapest
                                     1979 August 27




    W VIRGINIS STARS WITH PROPERTIES OF RV TAURI STARS





    In the present paper the notion of properties of RV Tauri
stars includes first of all a typical feature of their light
curves, i.e. the interchange of deep and shallow minima as well
as the instability of light curves. Many investigators pointed
to the light curve changes mainly as possible ones. Generally
these changes referred to the form. The knowledge on the 
difference of the light curves between odd and even cycles, i.e. the
alternation is much scanter. For the first time this phenomenon
was detected by Arp (1955, 1957) for two out of four observed
Cepheids of globular clusters with periods more than 17 days.
Later on Vasiljanovskaja and Erleksova (1966, 1969) considered
the question if it is possible to explain the photoelectric 
observational scatter of W Virginis stars in the galactic field
by the alternation. We found that explanation to be acceptable
for three stars. However, the scantness and the small interval
of observations left certain doubt in the reality of the alternation 
and did not enable to clear up its character. Finally
Stobie (1970) and Lloyd Evans (1970) demonstrated clearly that
SZ Mon, a classical cepheid according to GCVS 1969, showed the
alternation of the light curve. This alternation was expressed
in the difference of minimum magnitudes about 0.1m in V and
the form in two halves of the double period. These authors
classified SZ Mon as a unique Type II Cepheid, possessing 
properties of the RV Tauri stars. Publications on the W Virginis
star alternation are confined to the above works, as far as I
know.
    Our study of SZ Mon showed that during the whole observing
interval J.D. 2426710-43248 the light curve must be considered
with a double period. For all the 10 mean light curves the alternate 
minima differ by ~0.15m_pg in depth. The average amplitude
is 1.4m_pg. No systematic difference between the maximum magnitudes
has been observed, but on average the maximum following a deeper
minimum is fainter than the other one by ~0.05m_pg. In certain
rare cycles it is possible to level of the minimum magnitudes.
Asymmetry is about 0.16 of the double period in both halves.
SZ Mon should be considered as a W Virginis star both by a peculiar
light curve and a remarkable period change, as well as by a wide
loop (V, B-V) and moderate radial velocity.
    It turned out that SZ Mon is not an exception. Another
 Virginis star MZ Cyg must also be considered as having twice
its previous period. MZ Cyg observed for a long time and rather
intensively hid its alternation, i.e. the interchange of minimum
depths because on small telescopes its brightness is always measured 
together with two neighbouring stars. This reduces the
amplitude and distorts the real minimum magnitudes. Only the 40cm          
Zeiss photographic observations, representing estimates of the
brightness of the variable together with the South neighbour
which was fainter than 15m enabled to detect the alternation. Six
mean light curves in J.D. 2436730-43761 interval showed that as
in the case of SZ Mon the deep minimum fell on one and the same
phase and was deeper than the other one by 0.5-0.9 m_pg. The
amplitude of the light curve is 2.4 m_pg. The maximum following
the deep minimum is fainter than the other one by 0.2 m_pg.Thus
the relation between the maximum magnitude in MZ Cyg is contrary
to that most frequently found in RV Tauri stars. Asymmetry in
both halves is about 0.15 of the double period. Sometimes the
successive cycles can have equal maxima and equal minima.
    The preliminary conclusions considering the light curves
of MZ Cyg and SZ Mon are the following.
1. The alternation is of lasting character.
2. The resemblance of the RV Tauri light curve is restricted to
the difference of the minimum depths without their mutual 
transposition. The distinction becomes apparent in the shape and
greater stability as a whole.
3. The instability of individual cycles is sometimes possible
up to levelling successive cycles.

The companions of MZ Cyg and SZ Mon seem to be found among the
insufficiently studied W Virginis stars with large periods.
V420 Cen, RX Lib, CC Lyr, RS Pav and others can be supposed as
companion candidates. The presence of W Virginis stars with the
properties of RV Tauri stars agrees with the contemporary 
theoretical conception on the closeness of the evolutionary stages
of these stars.
    It is proposed to publish the detailed analysis of MZ Cyg
and SZ Mon light curves in Collected Articles "Variable Stars".



                        G. E. ERLEKSOVA

                     Astrophysical Institute
                     of Tadjik Academy of Sciences
                     734060 Dushanbe, U.S.S.R.




References :

Arp, H.C. 1955, A.J. 60, 1 [BIBCODE 1955AJ.....60....1A ]
Arp, H.C. 1957, A.J. 62, 129 [BIBCODE 1957AJ.....62..129A ]
Lloyd Evans, T. 1970, The Observatory 90, 254 [BIBCODE 1970Obs....90..254L ]
Stobie, R.S. 1970, M.N. 148, 1 [BIBCODE 1970MNRAS.148....1S ]
Vasiljanovskaja, O.P., Erleksova, G.E., Shakhovskaja, N.I.,
      1966, Tadjik Bull No. 48, 9
Vasiljanovskaja, O.P. and Erleksova, G.E. 1969, Mitteilungen
      Budapest-Szabadsaghegy, No. 65, 321 [BIBCODE 1969CoKon..65..321V ]
					[CoKon No. 65]