The nature of the variability

The mean (d, B₂-V₁)-values place both stars well within the $\delta$ Scuti instability strip as observed in the Geneva Photometric System. We note the interesting situation that two stars having such similar characteristics behave quite differently from the variability point-of-view. In the previous sections we have shown that the brightest component has a $\delta$ Scuti type of variability with a total amplitude of 0.05 mag while the fainter component presents no short-period variability of amplitude larger than 0.01 mag. What could the causes be for the different variability behaviour between both? From the Geneva colour indices, it appears that the brightest component has $\Delta d>0.100$, thus it is somewhat more evolved than its companion. Can we identify any pulsation mode for HD 220392? The physical parameters of Table 4 may be used for the computation of the pulsation constant Q:
log Q = log(f^-1) + 0.5 log($M/M_{\odot }$) + 0.3 M_{htmlcommentmark>73 bol} + 3 log( T_eff) -12.697.
The propagation of errors shows that the error on the pulsation constant is of order 0.003 days (0.07 on $\Delta (log Q)$). The results are given in Table 6. The values thus computed are on the high side for a definitive mode identification: the only conclusion one could draw is that the frequency f₂ possibly corresponds to the fundamental radial mode (F). The frequency ratio f₂/f₁, 0.84, here is not very helpful either. Additional photometric observations for this interesting couple of stars is highly recommended. The already obtained data are neither sufficiently numerous nor of sufficient quality to allow unambiguous solutions or to solve for the multiple frequencies. Radial velocities would be needed too. Because professional observatories no longer offer regular photometric facilities (especially in the southern hemisphere), we invite any seriously interested party that has the adequate instrumentation to an intensive but easy monitoring of this southern system.

Table 5: Q-values for HD 220392

Identifier	Frequency	log Q	Q	Comment
	[cpd]	[days]	[days]
HD 220392	4.674	-1.358	0.044 $\pm$ 0.003
	5.522	-1.431	0.037 $\pm$ 0.003	F?
	(6.522)	-1.501	0.032 $\pm$ 0.003	(F?)