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2 The observations

The observations presented in this paper were carried out at the European Southern Observatory (La Silla, Chile) from 14 to 29 February 1992. The 0.5 m ESO telescope, equipped with a single-channel photon-counting photometer, a thermoelectrically cooled Hamamatzu R-943/02 photomultiplier and standard ESO filters matching the
$UBV(RI)_{\rm c}$ system, was utilized. In order to obtain accurate differential photometry, for each program star (v) a comparison (c) and a check (ck) star were also observed (see Table 1). Each measurement of a star consisted in the average of 10-15 1-s integrations in each filter, according to the $U{-}B{-}V{-}R_{\rm c}{-}{\rm I}_{\rm c}$ color sequence. A complete observation consisted in sequential c-v-v-v-v-ck-c measurements. From these data, after accurate sky subtraction, four v-c and one ck-c differential magnitudes were computed; the four v-c values were finally averaged to obtain one data point. The observations were corrected for atmospheric extinction and transformed into the standard $UBV(RI)_{\rm c}$ system. The nightly atmospheric extinction coefficients were determined by observing two standards of very different spectral types in the 1-2.5 air mass range. Their mean values over the whole period are reported in Table 2.

Table 1: Comparison (c) and check (ck) stars for each program star (v), standard deviations $(\sigma)$ for the v-c and ck-c V-band differential magnitudes for each series of N nights, peak-to-peak $(\Delta V)$ amplitude of the V-band light curve and period (P) of variability

{\vert ll\vert l\vert l\vert c\vert c\vert c\vert c\vert l\vert}...
 ...ject{HD~157750} & \object{HD~158619} 
& 2 & 3 & 9 &.01&-- \\ \hline\end{tabular}

Table 2: Mean atmospheric extinction coefficients for La Silla site, obtained over the 14-29 February 1992 period

{\vert c\vert c\vert c\vert c\vert c\vert c\vert}
 & $U$\... \\ \hline
Mag/airmass &.538 &.308 &.203 &.165 &.118 \\ \hline\end{tabular}

These coefficients were obtained about eight months after the 1991 eruption of Mt. Pinatubo and are definitely higher than the values obtained with the same method and instrumentation in March 1991 (cf. Table 2 in Cutispoto 1998). Transformation coefficients were inferred by observing E-region standard stars (Menzies et al. 1989). The typical error of the differential photometry is of the order of 0.005 magnitudes, with somewhat larger values (up to 0.01 magnitudes) in the U-band due to the low photon counting level. The standard deviations ($\sigma$) for the v-c and ck-c mean differential V-band magnitudes obtained over N nights are reported in Table 1. The V magnitudes and colors of the comparison and check stars were obtained via standard stars (Menzies & Laing 1988; Menzies et al. 1989; Menzies et al. 1991) and are given in Table 3.

Table 3: V magnitude and colors for the c and ck stars derived from standard stars. Errors are of the order of 0.01 magnitudes. The symbol ":" denotes errors of the order of 0.02 magnitudes

{\vert l\vert c\vert l\vert c\vert c\vert c\vert\vert l\vert c\v...
 ....33:&1.26&0.63&1.20& HD~158619 &6.41&1.26:&1.19&0.60&1.11 \\ \hline\end{tabular}

For each program star the brighest V magnitude and corresponding colors are listed in Table 4. Taking into account the accuracy of the standard stars data and the extinction and transformation errors, the typical accuracy of the absolute photometry in Table 3 and 4 is of the order of 0.01 magnitudes, with somewhat larger values (up to 0.02 magnitudes) for the U-B colors.

Table 4: Maximum luminosity $(V_{\rm m})$ and corresponding colors measured for the program stars, inferred spectral classification (Spectral Type), distance range measured by the Hipparcos satellite $(D_{\rm H})$, photometric distance inferred from the adopted spectral classification $(d_{\rm ph})$ and V-band maximum luminosity ever observed $(V_{\rm max})$

{\vert ll\vert r\vert c\vert c\vert c\vert c\vert l\vert l\vert ...
 ...9 & 1.22 & K3~III + A5~V &
$207 - 402$\space & 358 & 8.23 \\ \hline\end{tabular}

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