COMMISSION 27 OF THE I. A. U.
               INFORMATION BULLETIN ON VARIABLE STARS
                            Number 1131

                                     Konkoly Observatory
                                     Budapest
                                     1976 May 4


        ON CHANGES OF PERIOD OF PULSATING STARS

   A loose correlation had been found by Vasiljanovskaya and
Erleksova (1968) between period and abrupt change of period for
Population II Cepheids. The changes of period we have found in
recent years at the Maria Mitchell Observatory for Mira and RR Lyrae
type stars seemed to fall nicely along the projections of this 
relation. I have therefore scanned the Remarks in the GCVS and its
Supplements in order to ascertain how general this relation is.
   In Figure 1 are plotted the values of log(DeltaP/P) against log P,
where DeltaP is the maximum difference between the various periods 
published for any given star. Stars with ostensibly constantly changing 
periods (JD_max = JD_0 + nP_0 +kn^2 ) are included the change of
period adopted in the Figure corresponding to a twenty-year interval 
from the epoch of P_0. For such stars it is frequently unclear

                         [FIGURE 1]

 Figure 1. Observed relationship between periods and extreme changes
 of period. Symbols: dots at right, Mira type; crosses, SR; triangles,
 W Virginis; open circles, classical Cepheids; dots at left, RR Lyrae
 stars. The one asterisk is for GG And, classified as alpha CVn type.


whether a progressive or sudden change of period is the more probable. 
Omitted are a few stars for which the original references 
indicated insufficient data to substantiate the indicated changes of
period. Stars with recorded Blazko effect, and those whose periods
appear to change cyclically as indicated by sine or cosine terms
were disregarded.
   The average values for each type of variable represented are
shown in Figure 2. The W Virginis stars and the delta Cepheids are each
divided into two groups, those longer and those shorter than ten
days. The RR Lyrae stars are separated into three groups: those
with sub-class ab (52 stars), sub-class c (7) and the ones for
which no sub-type is given (11). For comparison, the open circle
shows the average for the more thoroughly investigated galactic
RR Lyrae type stars published by Tsesevich (1972) and those in M5
studied by Coutts and Hogg (1969).

                          [FIGURE 2]

 Figure 2. Mean points for the various types of stars in Figure 1.
 The open circle represents the galactic RR Lyrae stars investigated
 by Tsesevich; the cross, those in M5 analysed by Coutts and Hogg.


                       DORRIT HOFFLEIT
                       Maria Mitchell Observatory
                       Nantucket, Mass., U.S.A.

References:
Coutts, C.M., Hogg, H.S. 1969, Pub. David Dunlap Obs. 3, 1-58 [BIBCODE 1969PDDO....3.....C ]
Tsesevich, V. 1972, Vistas in Astronomy, A. Beer, ed., 13, 241-256 [BIBCODE 1972VA.....13..241T ]
Vasiljanovskaya, O.P., and Erleksova, G.E. 1968, Non-Periodic
           Phenomena in Var. Stars, L. Detre, ed., Budapest,
           321-324 [BIBCODE 1969CoKon..65..321V ]