3 The UBV photometry

Nine photometric standard stars from the E1, E2, and E3 regions (Graham 1982) have been used for the determination of the transformation coefficients into the standard photometric system. UBV values from Grenon (1991) have been used in order to obtain a higher accuracy.

In addition, the A3 IV/V star HD 25653 from the E2 region has been used for measuring the first order exctinction in each photometric night.

In this night the mean (O-C) of the magnitudes of the exctinction stars in V was 0.004 mag, in B 0.006 mag, and in U 0.009 mag. The first order extinction coefficients that we obtained are the following:

$$k'_{v} = 0.22 \pm 0.01$$ (1)

$$k'_{b} = 0.30 \pm 0.01$$ (2)

$$k'_{u} = 0.56 \pm 0.02.$$ (3)

With the standard stars used, the transformation to the standard UBV system is given by

$$V = v + (0.102 \pm 0.006)\cdot(b-v) - (0.018 \pm 0.012)$$ (4)

$$(B{-}V) = (0.972 \pm 0.010)\cdot(b-v) + (0.012 \pm 0.015)$$ (5)

$$(U{-}B) = (1.066 \pm 0.017)\cdot(u-b) + (0.005 \pm 0.032).$$ (6)

The mean (O-C) value for V is 0.011 mag, for (B-V) 0.015 mag, and for (U-B) 0.030 mag. We present our photometric measurements in Table 1.
The first two columns of this table contain equatorial coordinates (J2000) of the primaries. Column three their HD number, if available. Column four the primary's spectral type. Column five the components magnitude difference $\Delta V$, column six $\Delta (B{-}V)$and column seven $\Delta (U{-}B)$.