2000A&A...362..245P

On the basis of almost 2000 b, y photoelectric observations of θ Tucanae collected in an international campaign at two sites and of almost 1500 v, u observations collected at one site, observational guidelines for mode identification have been searched. Not only the generally used plane (A_b–y/A_y versus φ_b–y-φ_y) but each combination of the nonadiabatic observables including amplitude ratio vs. amplitude ratio and phase difference vs. phase difference planes were checked for guidelines. Especially A_b/A_y amplitude ratio and φ_b–y - φ_b phase difference proved to be useful criteria for mode separation in the case of θ Tucanae. The 10 pulsational frequency values published previously by Paparo et al. (1996A&A...315..400P) are distributed in three groups according to A_b/A_y and in two distinct levels according to φ_b–y-φ_b. The groups consist of closely spaced frequencies. Group III (15.86, 15.94 and 17.06 cycles/day) displays a higher, distinct level than the others according to the A_b–y/A_y amplitude ratio and unusual behaviour in u colour. Although the discriminative plane does not give the exact value of quantum numbers but predicts which theoretical modes must share the same behaviour. Calibration of the pure observational guidelines by new, delicate theoretical calculation for planes of nonadiabatic observables concerning the location of theoretical modes is urged.