COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                    Number 2247

                                   Konkoly Observatory
                                   Budapest
                                   1982 December 10

                                   HU ISSN 0374-0676



      CYCLIC VARIATIONS OF THE PERIODS OF RR LYRAE TYPE STARS



   An investigation of the instability of the light variation period of
RR Lyrae type stars has been carried out at Odessa Astronomical Observatory.
From the investigated stars a group of objects could be distinguished for
which the presence of cyclicity in the variations of periods is inherent. It
was shown that the cyclicity observed is not due to the binary nature of the
objects studied.
  It has been supposed that the cyclicity in the variations of the periods
of RR Lyrae stars is caused by two different limited states of stars. The
nature of the internal mechanism which maintains the cyclic variations of the
periods is probable associated with variations in stellar structure.
(Firmanyuk, 1980; Astron. Circ. No. 1118)
  Cepheids, Mira-, RV Tauri- and eclipsing stars show cyclic variations in
their periods as well.
  At present the model of the mechanism maintaining cyclicity in the periods
of light variations of RR Lyrae stars has not yet been constructed. The
nature of cyclicity in the variations of the periods of variable stars of
other types is not clear either. Now it looks worth presenting the O-C
diagrams of some RR Lyrae type stars showing cyclic period changes.


                           [FIGURE 1]
                           [FIGURE 2]
                           [FIGURE 3]
                           [FIGURE 4]
                           [FIGURE 5]
                           [FIGURE 6]
                           [FIGURE 7]
                           [FIGURE 8]
                           [FIGURE 9]


The elements of these RR Lyrae stars are given below:

                     Table

Star     Elements                              Number
                                               of cycles

XX And   Max_Hel JD = 2428471.695+ 0.72275085d E   13000
         
UY Boo   Max_Hel JD = 2419803.827+ 0.65082987 E    14000
         
AE Boo   Max_Hel JD = 2430388.203+ 0.31489272 E    10000
         
Z CVn    Max_Hel JD = 2427624.236+ 0.65385618 E    22000
         
RZ Cep   Max_Hel JD = 2427614.434+ 0.30866588 E    18000
         
RV Cet   Max_Hel JD = 2429544.157+ 0.62337149 E    12500
         
RV CrB   Max_Hel JD = 2417823.330+ 0.3316351   E   18500
         
RW Dra   Max_Hel JD = 2426610.235+ 0.4429091   E   23000
         
SV Eri   Max_Hel JD = 2428398.202+ 0.7137491   E   22000
         
RR Gem   Max_Hel JD = 2422765.379+ 0.39728736 E    27500
         
AR Her   Max_Hel JD = 2424794.301+ 0.47000975 E    14500
         
WZ Hya   Max_Hel JD = 2427829.477+ 0.53773359 E    28000
         
CX Lyr   Max_Hel JD = 2418804.579+ 0.61666411 E    28000
         
BF Peg   Max_Hel JD = 2426589.301+ 0.49574816 E    16000
         
DH Peg   Max_Hel JD = 2441544.358+ 0.25548897 E    16500
         
TU Per   Max_Hel JD = 2415261.307+ 0.60707155 E    22000
         
RY Psc   Max_Hel JD = 2437910.402+ 0.52972595 E    23000
         
RV UMa   Max_Hel JD = 2933834.392+ 0.59412763 E    24000
         
BN Vul   Max_Hel JD = 2417861.4282+ 0.46806263 E   25000




            FIRMANYUK B.N.

            Odessa Astronomical Observatory
            U.S.S.R.