COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                     NUMBER 490

                                Konkoly Observatory
                                Budapest
                                1970 November 21


     THE SECONDARY PERIOD OF THE RRab STAR SZ Hydrae

      Inspired by a notice of L. J. Robinson (Per. Zv. V. 16.
No. 1., p. 62, 1966) on a possible Blashko effect in the light
variation of SZ Hya I initiated photoelectric observations
of the star during my stay at the Catania Astrophysical 
Observatory on Aetna in spring 1970 in order to determine the
length of its secondary period.
      The observations were carried out with the quasi-
Cassegrain reflector of 61 cm aperture and 600 cm focal
length, equipped with an EMI 6256 photomultiplier tube.
      Although only four maxima could be obtained, their
favourable distribution in phase enabled the determination
of the secondary period. The result is:

                  P_s = 25,8d
                   
Only Z CVn has a shorter secondary period among the RRab
stars (P_s = 22,75d, Inf. Bull. Var. Stars No. 146, 1966).

			[FIGURE 1]
			[FIGURE 2]

The table contains the observed maxima in two colours
(Delta)M_y, (Delta)M_b and their O-C values computed with
the following elements:

	Max.hel. = 2440879,412 + 0,53724022 E         (1)

Three photoelectric maxima obtained by Arizona observers
(Fitch, Wisnewski and H. L. Johnson: Comm. Lunar and
Plan. Lab. Numb. 71, pp 20-21, 1966) are included in the
table.

J.D. max.      O-C      (Delta)M_y  (Delta)M_b

               Arizona observations:

2438773,828   +0.007d   10,55m     10,80m
    810,884   -0.007d   11,14      11,04
    901,992   +0.0008   10,75      10,68

               Catania observations:
2440679,416   +0.004   -0,350     -0.780
    685,321   -0.001   -0.050     -0.420
    708,425   +0.002   -0.240     -0.640
    721,313   -0.004   +0.240     -0.080


     In figure 1 the values of Delta M_b and O-C are plotted
against the phase of the secondary period computed by the
formula:

       Max. amp. = 2440705 + 25,8d  N                   (2)

The Arizona observations indicate the same secondary period.
The Arizona magnitudes M_b were adjusted to the Catania
magnitudes by shifting them by -11,26m.
     Combining their observations with my Catania results,
an improved value of 28.74 can be derived for P_s, but the
number of elapsed epochs between the two series of observations
may be yet in error by +/-1.
     Fig. 2 shows the O-C diagram for the fundamental
period. Points denote the photographic epochs given by 
Robinson, the circles are visual epochs from Per. Zvj. (see
above), the triangles are the epochs obtained at Arizona
and Catania, respectively.
     I am very grateful to Prof. Godoli for a grant
enabling me to carry out observations in the Catania 
Observatory, and to Drs. C. Blanco, S. Catalano and M. Rodono
for helping me in the observations.



                         S. KANYO
                     Konkoly Observatory
                         Budapest