The Doppler method can be used to estimate the semimajor axis of a planet's orbit by: a. measuring the time it takes for the star's line-of-sight velocity to cycle from peak to peak, and using Newton's version of Kepler's Third law.
b. measuring the speed at which the star orbits the mutual center-of-mass of the star and planet, and using Newton's theory of gravity.
c. measuring the amount by which the starlight is reduced when the planet transits.
d. measuring the asymmetries in the velocity curve.