COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                     Number 2518

                                         Konkoly Observatory
                                         Budapest
                                         7 May 1984

                                         HU ISSN 0374-0676


      POLARIMETRIC OBSERVATIONS OF THE RED VARIABLE RV HYDRAE

  RV Hydrae (= HD 73766 = SAO 136144) is a SRc type variable star (spectrum
MS II :Keenan, 1942). Photographic magnitude is known to vary from 8.7 to
10.04 with a period of 116 days (Payne-Gaposchkin, 1954, Gaposchkin, 1956).
  Linear polarisation of RV Hya has been measured on 1984 March 4, 5 and 6
using the photoelectric polarimeter Sterenn on the 1 meter telescope at
Pic du Midi observatory (France). Unpolarized stars were observed to check
that no instrumental polarisation was present, and standard polarised stars
to calibrate the measurements. Three measurements through B filter and one
through V filter were performed (see table below).

     J.D.     filter    p (%)     Theta eq
     2445000+                       

     763.53    B     0.92+-0.08    88d+-2d
     764.49    B     1.06+-0.08    89d+-2d
     765.52    B     1.05+-0.10    89d+-2d
     765.52    V     0.53+-0.05    88d+-2d

Position angle theta eq is computed in the equatorial frame. B and V magnitudes
cannot be determined precisely. But it seemed that the star was near light
maximum.
  To the knowledge of the author, no linear polarisation measurement of
RV Hya has been published. Since the galactic latitude of RV Hya is 19d and
the stars in the neighbourhood are not strongly polarised, the polarisation
measured is probably intrinsic and not interstellar. Such a polarisation is
quite common among red variables. The decrease of polarisation from 1% in B
to 0.5% in V is in agreement with what is observed in polarised red variables
(see for example, Shawl, 1975). Usually polarisation varies with light 
intensity: more observations are needed.


                  J.F. LE BORGNE
          Observatories du Pic du Midi et de Toulouse
          14 Av. E. Belin - 31400 Toulouse - France

References:

Gaposchkin, S., 1956, Annals of the Astr.Observ. of Harvard College, 118, 2 [BIBCODE 1952AnHar.118....5G ]
Keenan, P.C., 1942, Astrophysical Journal, 95, 461 [BIBCODE 1942ApJ....95..461K ]
Payne-Gaposchkin, C., 1954, Annals of the Astr.Observ. of Harvard College, 113, 4 [BIBCODE 1954AnHar.113..189P ]
Shawl, S.J., 1975, Astronomical Journal, 80, 602 [BIBCODE 1975AJ.....80..602S ]