COMMISSION 27 OF THE I. A. U
        INFORMATION BULLETIN ON VARIABLE STARS
                     NUMBER 265

                                     Konkoly Observatory
                                     Budapest
                                     1968 March 31


   FLARES AND MEAN PHOTOMETRY OF YZ CANIS MINORIS


Six flares of YZ Canis Minoris have been observed photoelectrically in the blue
and ultraviolet at the 60-inch reflector of the Boyden Observatory during a
short monitoring run of 9 hours accumulated over 6 nights. These runs have the
distinction that they were preceded each night by an independent programme of
multicolour photometry which supplied reliable determinations of zero points in
the magnitude and colour equations. Mean magnitude and colours of YZ CMi in
the standard Johnson UBVR system have been derived. The largest source of
error in the results lies in the inexact knowledge of the form of the
transformations from instrumental to standard system for peculiar red stars
with emission line spectra, and the colour dependence at short wavelength of
the atmospheric extinction.

The 60-inch was equipped with the Dunsink photometer with a
magnetically-shielded E.M.I.9558 QA photomultiplier, cooled to 0dC by melting
ice, having a sensitivity extending from the ultraviolet to the infrared. The
following filters were used which remove the red leak and reproduce the
standard system of Johnson et al. (Ref. 1):

U 1mm UG 2 + 2.5mm 80% Saturated CuSO_4 soln. at 15dC.
B 1mm BG 12 + 2 mm GG 13 + 1 mm BG 18.
V 2mm GG 14 + 2mm BG 18.
R 2mm RG 5.

The effective wavelength of the red filter-tube combination at about 7150 A is
150 A longward of Johnson's value, but it cuts off the H-alpha line.

Table 1 gives the Universal Time of the coverage of YZ CMi and the observed
flares, together with the flares' approximate duration and magnitude range in
blue. Light curves for the larger flares in blue and ultraviolet are presented
in Fig. 1, where the flare intensity (delta J) is expressed in units of the
normal intensity of the star (J_0). Because of the very peculiar flare
continuum and line emission and the strange veiling of the underlying late-
type spectrum (Ref. 2), conversion to the standard UBV system has not been


                          [FIGURE 1]

attempted for the flares to avoid introducing false colour effects. In Fig.
2, the uncorrected instrumental colour, u-b, of the flares alone are shown with
a shift of the time-axis so that the moments of maximum blue light coincide.
The effects of normal differential extinction during these flares are
negligible. There is no evidence for a general trend in the u-b colour changes
of the present flares during their development, such as reported by P.
Chugainov in the colour index, B-V, where a reddening was observed immediately
following flare maxima (Ref. 3). The mean standard U-B of the total light
from star-plus-flare during decline is estimated at 0.^m12.

                          [FIGURE 2]


Table 1

1967         U.T.           Flares   Duration     Range
                                     (mins)      (blue)
Jan 6   0^h52^m- 2^h06^m   1^h46.^m8    6         1.^m23
    7   1 22   - 2  11
    8   0 07   - 1  08     0  40.1      7         0.30
        1 15   - 2  01
    9   0 30   - 1  06     0  44.8      2         0.15
        1 15   - 2  09     0  48.3     11         0.23
       23 54   -24  00
   10   0 00   - 2  01     0  44.6      2         0.38
   16   0 21   - 2  07     0  43.3      4         0.07

Observations of YZ CMi in four colours, before and after flares, were obtained.
These, together with similar observations of a nearby comparison star, were
reduced with the main photometric programme. Nightly-determined colour
equations and mean extinction coefficients, with second-order extinction terms
in V and B-V only, were used.

Table 2

YZ CMi                     Star a            Star b   Star c
V   11.^m28+-.^m003 (5)  12.^m46+-.003 (3)   12.^m47  10.^m72
V-R  1.90  +-.025  (10)   0.77  +-.015 (4)    0.80     0.21
B-V  1.60  +-.025  (11)   0.95  +-.026 (5)    1.13     0.51
U-B  0.91  +-.106  (10)   0.67  +-.054 (5)    0.88     0.10

In Table 2, mean standard magnitudes and colours, standard deviations from the
mean and the number of observations are given. No significant variations of YZ
CMi in its quiet phase were detected, except possibly in U-B which shows a
markedly higher standard deviation than that of the comparison star, and which
is three times higher than the standard error of the photometric programme.
Two further comparison stars were measured once, star c being useful for
photographic and visual estimates of flare amplitudes (see Fig. 3).


A.D. ANDREWS
Armagh Observatory

                          [FIGURE 3]

References:

(1) H.L Johnson et al., Comm. Of Lunar and Plan. Lab. No. 63 (1966). [BIBCODE 1966CoLPL...4...99J ]
(2) A.H. Joy, Stars and Stellar Systems Vol. 6, p. 668 (Univ Chicago 1960).
(3) P.F. Chugainov, Publ. Crimean Astroph. Obs. Vol. 33, p. 215 (1964).