One of the problems posed by the Cepheid pulsating concept is the extreme regularity of the period of variation and the disparities between radial velocities and magnitudes.

On the other hand, the high luminosity-period correlation by simple pulsation is not justified.

Also, of the smallest absolute magnitude by the stellar evolution model, if it is a single star.

On the other hand, eclipses are not observed if they were binary stars, unless they have a common atmosphere that prevents seeing such a phenomenon.

In giant stars so voluminous due to their great mass and F-G spectral type, the outer layers are very rarefied and allow interactive nuclei to be housed within them without their appearance revealing the binary condition of the system and affecting its stability too much.

The models based on the double nucleus allow such a possibility and present a good correlation with the observed variations. On the other hand, the periods and variations coincide with the behavior of binary nuclei, above all they justify the mass-period relationship, the most important and difficult to justify outside of a double system. For example: for Delta of Cepheus, the distance between nuclei would be 0.15 AU. while the common atmosphere approaches 0.2 U.A.; for X Cigni, the distance between nuclei would be 0.46 AU; and for T Monocerotis of 0.55 U.A. , within their respective common atmospheres.

In addition, regardless of their mass and period, they present at most a magnitude of difference or a little more without correlation with the period; attributable more to a variation in the circle-ellipse perspective, than to a variation in the volume of the pulsation, which should be much higher, especially in larger stars, as observed in very long-period variables.
As regards the rr lirae type variables, the problem is similar, although easier as they have similar absolute magnitudes. Their periods vary from 0.2 to 1.2 days. The model, if treated as stars of the main series, would give us distances between the stars of between 0.01 and 0.04 AU, with joint masses between 3.3 and slightly less than 6 solar masses, consistent with their common atmospheres. .

It is evident that statistics reinforce such an approach:

First.- 75 percent of the stars are binary.

Second.- About 20 percent are eclipsing, because they can be individualized since their spectral types are mainly 0-B-A and therefore denser, with less volume, which does not generate common atmospheres that hide the nuclei.

In such circumstances: can there not be binary stars hidden by common atmospheres, in short Cepheids...?

Carlos de Torres.