COMMISSION 27 OF THE I. A. U.
               INFORMATION BULLETIN ON VARIABLE STARS
                            NUMBER 69


                                             Konkoly Observatory
                                             Budapest
                                             24 September 1964


            SZ HYDRAE: AN RRab STAR WITH UNSTABLE PERIOD

      To extend G. Lange's unpublished listing of maxima later than
1929, SZ Hydrae was examined on more than 1 000 Harvard patrol
plates. Annual light curves (1899-1900 to 1951-1952) were plotted and
combined into an approximate mean light curve. This curve was then
fitted to each season's observations to determine the heliocentric
times of maxima given in Table I.

      All maxima to about 1930 are fitted by,

      J. D. Max._{hel} = 2 415 714.1429 + 0d.537 428 15 E.    (A).

      Later observations to 1952 showed large O - C residuals, 
requiring the elements:

      J. D. Max._{hel} = 2 431 881.5537 + 0d.537 230 51 E.    (B).

      Lange's observations from 1959 to 1962 (Ms.) show progressively 
larger positive residuals from B (0d.06 to 0d.12), indicating a
lengthened period.

      The O - C values show that the period of SZ Hydrae also has
slow oscillations, and rather large differences between successive 
residuals hint at short-term variations.

       The shape and amplitude of the light curve varied considerably
from season to season, as if a strong Blazko effect were present.

               TABLE I.

Hel. J. D.         E     O - C
---- --- --        -     -- -- A
  24...
15 064.943     - 1208   +0d.013
15 414.249        558    -0.009
15 714.143          0     0.000
16 170.956      + 850    -0.001
16 539.089       1535    -0.006
16 886.269       2181    -0.005
17 216.243       2795    -0.012
17 626.850       3559     0.000
17 981.015       4218     0.000
18 351.844       4908    +0.004
18 716.228       5586    +0.011
19 467.013       6983    +0.009
20 195.782       8339    +0.026
20 561.231       9019    +0.024
20 923.453       9693    +0.019
21 278.179      10353    +0.042
21 658.157      11060    +0.059
21 995.111      11687    +0.045
22 391.727      12425    +0.039
22 670.642      12944    +0.029
23 119.963      13780    +0.060
23 477.323      14445    +0.030
23 841.173      15122    +0.042
24 224.348      15835    +0.030
24 578.514      16494    +0.031
25 309.387      17854    +0.002


                  TABLE I. cont.

Hel. J. D.         E      O - C
----------         -      -----A
 24...
25 668.401     +18522   +0d.014
25 977.437      19097    +0.029
26 410.575      19903     0.000
26 763.611      20560    -0.055
27 124.180      21231    -0.100
27 493.386      21918    -0.107         E       O - C
27 884.024      22645    -0.179         -       -----B
28 234.321      23297    -0.286       -6789    +6d.025
28 600.135      23978    -0.460        6108     -0.015
28 933.236      24412    -0.603        5488     +0.003
29 327.026      25331    -0.709        4755     +0.003
29 693.974      26014    -0.825        4072     +0.023
30 047.993      26673    -0.971        3413     +0.007
30 416.004      27358    -1.098        2728     +0.015
30 777.526      28031    -1.265        2055     -0.019
31 164.339      28751    -1.401        1335     -0.012
31 522.692      29418    -1.512         668     +0.008
31 881.563      30086    -1.643           0     +0.009
32 276.402      30821    -1.814        +735     -0.016
32 605.210      31433    -1.912        1347     +0.007
32 966.244      32105    -2.030        2019     +0.022
33 343.348      32807    -2.200        2721     -0.010
33 674.309      33423    -2.295        3337     +0.017
34 052.520      34127    -2.433        4041     +0.018


                     [FIGURE 1]


This diagram shows the residuals from elements "A". Only maxima
determined from Harvard patrol plates are plotted. Dot size indicates
relative weight.


                                L. J. ROBINSON
                              "Sky and Telescope"
                               49 Bay State Rd.
                              Cambridge 38, Mass.
                                    U. S. A.