Estimation of the Parameters of the Spiral Pattern in the Galaxy Based
  on a Sample of Classical Cepheids
  We consider a sample of Galactic classical Cepheids with highly accurate
estimates of their distances taken from Skowron et al., where they were
determined based on the period-luminosity relation. We have refined the
geometric characteristics of two spiral arm segments-the Carina-Sagittarius and
Outer ones. For this purpose, we have selected 269 Cepheids belonging to the
Carina-Sagittarius arm with ages in the range 80-120 Myr. From them we have
estimated the pitch angle of the spiral pattern $i=-11.9\pm0.2^\circ$ and the
position of this arm $a_0=7.32\pm0.05$ kpc for the adopted $R_0=8.1\pm0.1$ kpc.
In the Outer arm we have selected 343 Cepheids with ages in the range 120-300
Myr. From them we have found $i=-11.5\pm0.5^\circ$ and $a_0=12.89\pm0.06$ kpc.
Adhering to the model of a grand-design spiral pattern in the Galaxy with one
pitch angle for all arms, we can conclude that this angle is close to
$-12^\circ$.
