COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                      Number 918

                                    Konkoly Observatory
                                    Budapest
                                    1974 August 21


           ON THE VARIABILITY OF Pi PsA

    Pi PsA was pointed out to be a Cepheid variable by Strohmeier
et al. (IBVS 86, 1965), who assigned a period of 7d.975 and a small
amplitude (A_{pg}=0m.3). Kukarkin et al. (GCVS 1970) still quote these
characteristics in the edition of the GCVS. Buscombe and Morris
(MN 123, 183, 1961) found the star to be a single-lined spectroscopic 
binary with p = 356d.567, and Gliese's (Heid.Ver. 22, 1969) data
read as follows:

V= 5m.10,      B-V = + 0m.29,   (U-B)_c = + 1m.56
mu_{alpha} = 0".071, mu_{delta} = 0".085, pi = 0".050 +- 0".010 (p.e.)
Spectral type : F0 IV - F0 V (IBVS 680, 1972)

    New observations by Bopp et al. (MN 147, 355, 1970) led to a 
revision in star's orbital period : P = 178d.3177 +- 0d.0038 (p.e.). The
measured gamma velocity was 6 km/s. Gliese's proper motion data were
retained, and for the parallax a value pi = 0".044 was given. These
authors found systematic deviations of old radial velocity measures
from the computed curve, which they suggest may indicate the presence 
of a third body in the system.
    More recently, Petit (IBVS 680 and 695,1972) argued against
the Cepheid nature of the star by saying that it could hardly be a
delta Cephei variable if the proper motion and luminosity class were to
be confirmed. He has also mentioned the recent UBV observation by
Corben et al. (MNASSA 31, 7, 1972), which agrees with Gliese's data
and led to U - B = - 0m.01, thus confirming the above quoted (U - B)_c
value (see Cousins and  Stoy, (RO Bull. 64, 1963), for the (U-B)-(U-B)_c
relation).
    Pi PsA was included in a program of five-color photometry (Lick's
UVBGR, ApJ 98, 20, 1943) of southern Cepheids performed during 1970 at
the ITA Astronomical Observatory S. Jose dos Campos, Brazil. No variability 
has been detected within 0m.02 in light and color, and the UBV
data has been confirmed (Astr.Aph. 1974, in press).
    Our intention here is to throw some more light on the problem,
and to call attention on some properties of this star which, we think
deserves further examination.

    From Bopp et al's parallax we deduce that the star is at a
distance of 23 pc, which gives M_v ~ + 3m.3 +- 0m.5 assuming an error
of +- 0".010 in pi. In comparing its M_V, (B - V) and (U - B) values with
the new standard data of Allen (Astr.Quant. 3rd ed. 204 and 206, 1973)
for main-sequence stars, we note that:

    a) The (B - V) index and the absolute magnitude are typical
for a F1V unevolved object.

    b) The star has a (U - B) excess of about 0m.06.

    Pi PsA figures in Lindeman and Hauck's (Astr.Aph.Suppl 11, 119,
1973) {uvby beta} catalogue with the following characteristics:

 b - y = 0m.200, m_1 = 0m.159, c_1 = 0m.690 and beta = 2m.741.

From these, some conclusions can be reached:

    1) Its position in the {m_1} - {c_1} diagram by Stromgren (Ann.
Rev.Astr.Aph. 4, 433, 1966) is in good agreement with the assigned F0V
spectral classification. The star lies near the ZAMS line in Crawford
et al's (Astr.Aph.Suppl. 5, 109, 1972) (b - y) - c_1 plot.

    2) Its beta, (b - y) and c_1 values are typical of A star, according
to the calibration by Crawford (IAU Coll. "Stellar Rotation" 204, 1970).
The metallic line index m_1 indicates, however, metal-content deficiency 
relative to the Hyades stars (delta-m_1 = + 0m.022), confirming the
(U - B) excess referred to above.

    3) From Crawford's M_v (beta) calibration one gets M_v ~ 2m.7 (rms=
=0m.3), which agrees statistically with the values calculated from 
parallax measures.

    4) The iron-hydrogen ratio can be estimated from the m_1 - {Fe/H}
relation by Stromgren (In Vol 3. Ch. IX of Stars and Stellar Systems,
1963), for F-to-late G main sequence stars. For Pi PsA, one gets {Fe/H}=
= 0.076 (rms = 0.15). Thus, in terms of metal-content one can also
verify that the star in far from having Cepheid characteristics.

    If the star's variability is real (the only reference about it
being that of Strohmeier et al.), the amplitude, absolute magnitude and
colors would rather be suggestive of delta Scuti or AI Velorum "dwarf
Cepheids" types. Baglin et al's (Astr.Aph. 23, 221, 1973) data on these
short-period variables can help to test the hypothesis. Some brief 
conclusions are reached:

- from the P - Delta-C_1 relation one might expect for Pi PsA
(delta-c_1 - 0m.03) a fundamental period of about 0d.05.

- it is near the instability strip in the M_v - (b-y) diagram, if we
take M - 2m.7, as suggested by {uvby beta} data.

- its metallic line index m_1 is somewhat smaller than that expected
for a main sequence delta Scuti object, as can be seen from the m_1-(b-y)
plot.

- the observed discrepancies by Bopp et al. in radial-velocity curves
could be explained by beat phenomena and variations of amplitudes,
frequently present in delta Scuti stars.

    It seems, therefore, most probable that Pi PsA is not a classical
Cepheid variable. New accurate observations are needed to check a 
possible small-amplitude variation (a few hundredths of magnitude, at
most), to solve the differences in the M_v calculations and to determine
the nature of the systematic discrepancies of radial velocity measurements 
mentioned by Bopp et al.


Sao Paulo, maio de 1974


                                        E. JANOT PACHECO
                               Instituto Astronomico e Geofisico
                                     Universidade Sao Paulo