COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1603

                                     Konkoly Observatory
                                     Budapest
                                     1979 May 14



               VX PUPPIS - A PUZZLE




    About a dozen cepheids with periods between 1.5 and 6.3 days
are known to have beat periods such that the ratio of secondary
to primary period is close to 0.7 (e.g., Faulkner 1977). In most
of the cases for which both light curves have been published,
the secondary appears normal, in the sense that ascending light
is steeper than descending. Stobie's (1970) curves for the 
secondary period of VX Puppis, to the contrary, show a markedly
steeper descent. The periods given are P_1=3.01172d and P_2=2.1370d,
the latter having been found by first assuming a ratio close to
0.7. The curve, however, looks spurious, and on the basis of the
same 17 sparse published observations used by Stobie (Mitchell
1964) I was able to obtain another period, P_3= 1.8706d for which
the light curve is normal. The two periods are related by
           1/P_3 = 1/Sid.Day - 1/P_2.
    The visual light curves are shown in the Figure, where the
upper plot shows the original observations represented by P_1.
The second and third strips show how well the deviations of the
observations from the smooth curve are represented by P_2 and P_3,
respectively. An additional 12 later photoelectric observations
by Takase (1969) roughly substantiate these periods.
    From an observational standpoint one would intuitively favor
P_3 over P_2; however, the ratio of P_3/P_1 is only 0.621 whereas
theory (Stobie 1969) favors 0.7. That the primary period might be
spurious as well is not likely. C.H. Payne-Gaposchkin (1952) 
published a period of 3.01209072d based on 624 Harvard plates spanning
about half a century. She made no mention of a secondary or beat
period. Since the amplitudes Stobie found for the primary and
secondary are comparable, the Harvard material should reveal beat
phenomena. The GCVS does indicate that Russian astronomers have
found an indication of a 10d beat period. This is not obviously

                         [FIGURE 1]

 Light curves for VX Pup. Upper, Observations represented by
 P1=3.01172d. Middle, Residuals from the smooth curve above fitted
 to P2=2.1349d. Lower, Residuals represented by P3=1.8706d. Markers
 at intervals of 0.2 period and 0.2 mag.

consistent with the periods given here. If the beat period were
15d we would have very closely, 3P_1= 7P_2= 8P_3= 15.0, with no
clear choice between P_2 and P_3.
Obviously this interesting star merits further investigation
in order to resolve a possible conflict between observation and
theory.



                          DORRIT HOFFLEIT

                          Department of Astronomy
                          Yale University
                          New Haven, Conn. 06520
                          U.S.A.

References :
Faulkner, D.J. 1977, Ap.J. 216, 49 [BIBCODE 1977ApJ...216...49F ]
Mitchell, R.I.,Iriarte, B.I., Steinmetz, D. and Johnson, H.L. 1964,
       Tonantzintla-Tacubaya Bull. 3, 153 [BIBCODE 1964BOTT....3..153M ]
Payne-Gaposchkin, C.H. 1952, Ann.Harvard Coll.Obs. 115, No. 18 [BIBCODE 1952AnHar.115..205P ]
Stobie, R.S., 1969, M.N.R.A.S. 144, 511 [BIBCODE 1969MNRAS.144..511S ]
Stobie, R.S., 1970, Observatory, 90, 20 [BIBCODE 1970Obs....90...20S ]
Takase, B., 1969, Tokyo Ast.Bull. No. 191 [BIBCODE 1969TokAB.191.2233T ]