... approximation (QSA)

We recall that a sufficient condition for the validity of the QSA is that the atmosphere of the pulsating star can be described, at any time, by a classical hydrostatic, plane parallel model in radiative/convective equilibrium and local-thermodynamic-equilibrium (LTE), identified by the effective temperature $T_{\rm eff}$ and by the effective gravity $g_{\rm eff}= \frac{G M}{R^2}+ \frac{{\rm d}^2 R}{{\rm d}t^2}.$

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... models

Canonical models are the ones constructed by adopting a mass-luminosity relation based on evolutionary computations which neglect convective core overshooting during hydrogen burning phases (Castellani et al. 1992).

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... and "O''

We tried to determine the radius for the few Cepheids with known orbits, by disentangling orbital motions and pulsation in the radial velocity curve, but the error produced in this procedure remained still too high to secure good results.

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