3 The light curves

The light curves of TY$\,$CrA in the instrumental system are presented in Tables 2a-2d, containing 698 points from 1989 (Table 2a), 135 points from 1992 (Table 2b), 371 points from 1993 (Table 2c), and 585 points from 1994 (Table 2d) in each colour. Typical rms errors of one magnitude difference (calculated from all points and their deviation from a third order spline interpolation curve used in the calculation of the normal points, see Vaz et al. 1995) are $0\hbox{$.\!\!^{\rm m}$}006$(u,v) and $0\hbox{$.\!\!^{\rm m}$}005$ (b,y). Most phases were covered at least twice in the periods 89, 92-93 and 94.

  

Figure 1: y magnitude differences (instrumental system) between TY$\,$CrA and HD$\,$176$\,$423 in 1989, 1992 and 1993

  

Figure 2: y magnitude differences (instrumental system) between TY$\,$CrA and HD$\,$176$\,$423 in 1994

The y light curves TY$\,$CrA - HD$\,$176$\,$423 derived from the 1989, 1992 and 1993 observations are shown in Fig. 1, phased according to the ephemeris presented in Paper III:

Min I at:	HJD 2449537.8414	+ $2\hbox{$.\!\!^{\rm d}$}888779$	E.
	$\pm$8	$\pm$13

As discussed in Paper III, the period is variable, with minima occurring up to 0.004 days from those predicted by the above ephemeris. This indicates that orbit of the tertiary star extends some $\pm$0.35 AU along the line of sight, possibly more.

The y light curve from 1994 is shown in Fig. 2. The colour index curves from the 1992 and the 1993 data are shown in Fig. 2 of Paper III, and the colour index curves for the 1994 data are shown here in Fig. 3 (the larger uncertainties of the 1989 data do not justify a more detailed presentation of the colours curves). The larger measurement errors in u are apparent in the scatter of the (u-y) index curve.

  

Figure 3: b-y, v-y and u-y colour index differences (instrumental system) between TY$\,$CrA and HD$\,$176$\,$423 in 1994. The vertical dotted lines mark the phase limits of both eclipses, according to the theoretical solution of Paper III

 

Table 3: Transformation coefficients for calibration of the instrumental system to the standard uvby system, for years 1993 and 1994

Coefficients for transformation to the standard system are presented in Table 3, for the 1993 and 1994 observing runs, respectively. They should be used with the instrumental magnitude differences (TY$\,$CrA-HD$\,$176$\,$423, Tables 2a-2d) according to the equations (Olsen 1983):

$$\Delta V_{\rm std}=\Delta y_{\rm intr}+{\rm B} \Delta (b-y)_{\rm std}$$ (1)

$$\Delta (b-y)_{\rm std}={\rm D}\Delta (b-y)_{\rm instr}$$ (2)

$$\Delta m_{\rm 1,std}={\rm F}\Delta m_{\rm 1,instr}+ {\rm J}\Delta (b-y)_{\rm std}$$ (3)

$$\Delta c_{\rm 1,std}={\rm H}\Delta c_{\rm 1,instr}+{\rm I} \Delta (b-y)_{\rm std}.$$ (4)

Following Olsen (1983), these transformation coefficients have been determined using standard stars of spectral type F and earlier, i.e. $(b-y)_{\rm std} < 0.41$, a domain which is valid for both the comparison stars and for TY CrA itself, which is dominated by the light of the early-type primary star.