COMMISSION 27 OF THE I.A.U.
                INFORMATION BULLETIN ON VARIABLE STARS
                            Number 2987

                                                 Konkoly Observatory
                                                 Budapest
                                                 20 February 1987

                                                 HU ISSN 0374 - 0676


        VW HYA - ULTRAVIOLET ECLIPSE OF THE SECONDARY
                     AND NEW EPHEMERIS

The eclipsing binary VW Hya (not in HD, BD, CPD, discovery 
number 72.1929) was discovered by Hoffmeister (1929) as
an eclipsing binary with a very deep minimum, with a brightness 
outside minimum of 10.5 magnitude. The star has been
observed with the 70cm reflecting Swiss telescope at La
Silla Observatory in Chile with the P7 photoelectric photometer 
(see Burnet 1976 and Burnet, Rufener 1979) in the
Geneva photometric system. The observations continue and we
publish the preliminary results because of the rather
unusual behaviour of the system during secondary eclipse.
   Generally the shape of the light curve is typical of a
double star with red and blue components, however the secondary 
eclipse in the U colour is unexpectedly too deep.
Obviously a source of ultraviolet radiation is eclipsed
during the secondary eclipse. Fig. 1 shows the light curve
in the V filter. Fig. 2 shows the detailed secondary minimum
in the U filter. Measurements in the B1 filter, which is
closest to U, do not show the secondary minimum - Fig. 3.
For readers not familiar with the Geneva photometric system,
Fig. 4 shows the response curves for filters U and B1 (see
Rufener 1963, 1981 or Golay 1974, 1980).
   The observation is based on only one observation of the
secondary minimum. The effect may be transient and observers
are advised to monitor the secondary eclipse in colour U.
Obviously spectroscopic observations may finally reveal more
details concerning UV emission on the secondary star. The
new ephemeris is as follows:

       T(minI) = 2446083.7665 + 2.6964378.E   (in HJD)

   These are provisional results. More details will be
published later after reduction of all observations for a
complete light curve.

Coordinates 1950.0: RA = 8:31:31, Dec. = -14:29.8.


                    [FIGURE 1]

   Fig. 1. Light curve of VW Hya. Magnitude [V] against phase.
   Filter [V] of the Geneva photometric system corresponds to V
   of the UBV system.

                    [FIGURE 2]

   Fig. 2. Secondary minimum of VW Hya in filter [U].
   Ordinates: Magnitude [U]. Abscissae: Phase.

                    [FIGURE 3]

   Fig. 3. Secondary minimum of VW Hya in filter [B1].
   Ordinates: Magnitude [B1]. Abscissae: Phase.

                    [FIGURE 4]

   Fig. 4. Response curves for filter [U] and [B1]. For
   comparison response curve of filter U from UBV system as
   dashed line.

                    [FIGURE 5]

   Fig. 5. Finding chart for VW Hya. Two brighter nearby stars
   marked a and b are marked on the chart too. Star a is
   HD72530, V=9.14 mag. and b is HD72462 V=6.38 mag. The
   chart is based on the atlas by Papadopoulos (1979).



          Z. KVIZ*, F. RUFENER

         Observatoire de Geneve
           CH-1290 Sauverny
            Switzerland


* On leave from:
  School of Physics, University of New South Wales, P.O.Box 1,
  Kensington, NSW, Australia.


References:

Burnet M. 1976, Thesis No. 235, Ecole Polytechnique Federal de Lausanne. [BIBCODE 1976PhDT.......217B ]
Burnet M., Rufener F. 1979, Astron. Astrophys. 74, 54. [BIBCODE 1979A&A....74...54B ]
Golay M., 1974, Introduction to Astronomical Photometry, Reidel, Holland. [BIBCODE 1974ASSL...41.....G ]
Golay M., 1980, Vistas in Astronomy, 24, 141. [BIBCODE 1980VA.....24..141G ]
Hoffmeister C. 1929, AN 236, 233. [BIBCODE 1929AN....236..233H ]
Papadopoulos C. 1979, True Visual Magnitude Photographic Star Atlas, Pergamon
    Press. [BIBCODE 1979tvm2.book.....P ]
Rufener F. 1963, Archives des Sciences, Geneve, Vol. 16, fasc. 3, p. 413 =
    Publ. Obs. Geneve, ser. A, 66 (1964).
Rufener F. 1981, Astron. Astrophys. Suppl. Ser., 45, 207. [BIBCODE 1981A&AS...45..207R ]