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1 Introduction

Symbiotic stars are binary systems composed of a cool giant and a hot, luminous white dwarf. They show variability over any time scale from minutes (flickering) to several decades (outbursts of symbiotic novae), with phenomena related to the orbital motion having periodicities generally between 1 and 4 years (or a few decades in the systems harbouring a Mira variable, $\sim$20% of all known symbiotic stars).

Such long time scales tend to discourage stand-alone photometric campaigns from a single Observatory (which would require observational programs running up to $\sim$10 years in order to derive - for example - a firm orbital period). Most of the current photometric investigations of symbiotic stars therefore try to assemble as much as possible data from the widest set of current and archival sources. A template example is the recent reconstruction of the 1890-1996 lightcurve and orbital period determination for YY Her by Munari et al. (1997).

The data so collected are generally very heterogeneous in nature, with large differences caused for example by (a) the non-standard photometric bands, (b) the adopted comparison sequences and standard stars, (c) lack of adherence to and transformation into a system of general use, like the $UBV(RI)_{\rm C}$, and (d) telescope focal length or pixel scale that causes blending with images of nearby field stars. These differences generally may introduce such a large scatter in the data that all but the strongest details are washed out.

The establishment of suitable and accurate photometric comparison sequences covering a wide range in magnitude and colors should alleviate considerably some of the above problems, and could encourage small observatories and/or occasional observers to obtain new data as well as to encourage those with access to plate archives to search for valuable historical data.

To this aim we present here suitable, $UBV(RI)_{\rm C}$ comparison sequences for 20 symbiotic stars (all but a few accessible from both hemispheres. See Table 1 for a list of the program stars). The sequences are basically intended to allow a general observer to capture on a single CCD frame or to have in the same eyepiece field of view when inspecting archival photographic plates: (a) enough stars to cover the whole range of known or expected variability for the given symbiotic star, (b) stars of enough different colors to be able to calibrate the instrumental color equations and therefore reduce to the standard $UBV(RI)_{\rm C}$ system the collected data, and (c) stars well separated from surrounding ones to avoid blending at all but the shortest telescope focal lengths.

Table 1: List of program symbiotic stars. The coordinates for the symbiotic stars are from our observations (equinox J2000.0, mean epoch 1999.5). The $e_\alpha $ and $e_\delta $ columns list the errors in milliarcsec for right ascension and declination, respectively. The last two columns list the coordinates of the field centers in Figs. 1 and 2
{\psfig{,width=18cm} }\end{table}

  \begin{figure}{\psfig{,width=17.5cm} }
\end{figure} Figure 1: Finding charts for the $UBV(RI)_{\rm C}$ comparison photometric sequence around the program symbiotic stars. The fields are in the same order as in Table 1. North is up and East to the left, with an imaged field of view of 5.16 $\times $ 5.16 arcmin and a 5.4 $\times $ 5.4 coordinate grid (see bottom-right panel of Fig. 2). Stars are plotted as open circles of diameter proportional to the brightness in the V band. The stars making up the photometric sequence (see Table 2) are plotted as filled circles

  \begin{figure}\par {\psfig{,width=17.5cm} }
\end{figure} Figure 2: Same as Fig. 1

  \begin{figure}{\psfig{,width=17.5cm} }
\end{figure} Figure 3: Comparison stars bright enough to cover the outburst phases lay outside the fields of Figs. 1 and 2 for eleven symbiotic stars. They are identified in these wider finding charts, where the portion plotted in greater detail in Figs. 1 and 2 is outlined by a dashed square. The symbols are the same as in Figs. 1 and 2, with an imaged field of view of about 11.4 $\times $ 11.4 arcmin and a 12 $\times $ 12 arcmin coordinate grid. To avoid overcrowding, the limiting magnitude is much brighter ($V \sim $ 16 mag) than in Figs. 1 and 2

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