A binary star system consists of two stars, $P$ and $Q$, orbiting their common centre of mass. An observer in the plane of the orbit views the system. At a particular instant, star $P$ is moving directly towards the observer with speed $v$, and star $Q$ is moving directly away from the observer with speed $v$ (where $v\ll c$).

A spectral line has an emission wavelength ${\lambda }_0$ and frequency $f_0$ in the rest frame of the stars.

Which row correctly describes the observed wavelength ${\lambda }_P$ from star $P$ and the observed frequency $f_Q$ from star $Q$?