2 Instrumentation and data quality

All three telescopes are located at Fairborn Observatory near Washington Camp in southern Arizona and operate fully automatically (see http:// 24.1.225.36/ fairborn.html for a description of the observatory).

2.1 The two 0.75-m APTs "Wolfgang'' and "Amadeus''

The Wolfgang APT is optimized for blue wavelengths with an EMI-9124QB photomultiplier and Strömgren by filters while Amadeus is optimized for red wavelengths with an EMI-9828 tube and Johnson-Cousins $V(RI_{\rm C}$) filters (for details we refer to Strassmeier et al. [1997b]). All measurements were made differentially between the variable and a comparison star (the latter is sometimes referred to as the "Comp'' star). A check star is used to verify the stability of the comparison star (we sometimes refer to it as the "Check'' star). Stars brighter than $\approx$5$^{\rm m}$ were measured with a neutral-density filter in front of the spectral filter. Integration time was usually set to 10 s except for stars fainter than $8-9^{\rm m}$ where 20-30 s for the broad-band filters and 30-60 s for the intermediate-band filters were used. The data reduction was based on nightly extinction coefficients obtained from a set of standard stars or, for the nights when an insufficient number of standards was observed (less than 20), an average of the previous three good nights was adopted. Otherwise, the procedures were the same as described in Strassmeier et al. ([1997a]).

After the obviously deviant data points due to misidentifications and clouds were eliminated, we computed external uncertainties, $\sigma_{\rm ext}$, for all check-minus-comparison magnitudes. Such uncertainties allow an examination of the long-term data quality expected for the variable-minus-comparison data. The mean external standard deviation of a "nightly mean'' from a yearly mean was for Wolfgang 4 millimag in b and y, and for Amadeus 6, 8, and 10 millimag in V, R, and I, respectively. The mean internal standard deviations from three readings of the variable and four readings of the comparison star were less than 2 millimag for Wolfgang and approximately 4 millimag for Amadeus. Between May and June 1997, the Amadeus APT had gotten repeatedly out of focus and the internal standard deviations raised to 7-8 millimag in V during this time.

2.2 The Phoenix-10 0.25 m APT

The Phoenix-10 APT is now located at the same site in Washington Camp as Wolfgang-Amadeus and thus follows the same weather and extinction pattern. It is already in routine operation since 1983 and is managed by Mike Seeds at Franklin & Marshall College as a multi-user telescope (see "Phoenix-10 Newsletter'' and Seeds [1995]). Strassmeier & Hall ([1988a]) examined its data quality from its first four years of operation and found external uncertainties of 10, 20, and 28 millimag in V, B, and U, respectively. For the stars in this paper, integration times were set to 10 s for all targets. More recently, Henry ([1995]) compared the long-term external precision of the Phoenix-10 APT with APTs of larger aperture (the Vanderbilt/Tennessee State 0.4 m and the Tennessee-State 0.8 m) and verified the telescope's long-term stability. A further comparison was made with Wolfgang and Amadeus as well as with the University of Catania 0.8 m APT on Mt Etna (Strassmeier et al. [1997a]). All relative zeropoints in the V bandpass agreed to within their formal errors. In order to eliminate datapoints grossly in error, we applied a statistical procedure that excluded all data with an internal standard deviation greater than 20 millimag in V.