2 Observations

TY$\,$CrA was observed in the Strömgren uvby system with the SAT reflecting telescope at ESO, La Silla, Chile, using the six-channel spectrograph-photometer and photon counting system described by Nielsen et al. (1987). Observations were obtained during 22 nights from June 14 to July 9, 1989 (by IH), 5 nights from June 8 to 14, 1992 (JA), 19 nights from May 26 to June 27, 1993 (LPV, BWC, JA) and 32 nights from June 05 to July 15, 1994 (LPV). A circular diaphragm of $17^{\prime\prime}$ diameter was used throughout.

Sky background measurements were taken systematically at a position $60^{\prime\prime}$ N and $60^{\prime\prime}$ E from TY$\,$CrA. However, these did not correct for the contribution of the reflection nebulae NGC$\,$6726/7. Correction for the nebular light is discussed in detail in Paper III, where the nebula is found to contribute roughly one third of the observed flux in the 17$\hbox{$^{\prime\prime}$}$ diaphragm at quadrature in all of the colours.

Extinction corrections were based on the nightly coefficients from the two comparison stars and other constant stars. When needed, linear or quadratic corrections for instrumental drift (uncooled photomultipliers) and/or changes in sky transparency during the night, determined from repeated measurement of several constant stars, were applied. HD$\,$176$\,$423 and HD$\,$176$\,$497, both within $0\hbox{$.\!\!^\circ$}6$ of TY$\,$CrA, were used as comparison stars and observed alternately between the measurements of TY$\,$CrA. Both stars were found to be constant within the observational accuracy throughout the four observing periods. The observational mean errors of a magnitude difference between the comparison stars (data from 1992, 1993 and 1994; the errors in 1989 were roughly a factor 1.2 higher) were:

$${\sigma(\Delta u) = 0\hbox{$.\!\!^{\rm m}$}008,~~~~~\sigma (\Delta v) = 0\hbox{$.\!\!^{\rm m}$}007,} \atop$$

$${\sigma(\Delta b) = 0\hbox{$.\!\!^{\rm m}$}007,~~~~~\sigma (\Delta y) = 0\hbox{$.\!\!^{\rm m}$}006.}$$

Information and photometric results for both TY$\,$CrA and the comparison stars are given in Table 1. The photometric indices for TY$\,$CrA have not been corrected for nebular contamination. While three stars contribute to the flux from the system, both the primary/secondary and primary/tertiary luminosity ratios are greater than 30. Thus the photometric indices (once corrected for the nebular contamination, see Paper III) primarily correspond to the photosphere of the primary star.

  

Table 1: Data for TY$\,$CrA and the comparison stars. The uvby indices for TY$\,$CrA correspond to phase $0\hbox{$.\!\!^{\scriptscriptstyle\rm p}$}349$ (1994), while its $\beta$ index was obtained at $0\hbox{$.\!\!^{\scriptscriptstyle\rm p}$}625$ (1993)