Polaris , the nearest and brightest classical Cepheid , is a single-lined spectroscopic binary with an orbital period of 30 years .
Using the High Resolution Channel of the Advanced Camera for Surveys onboard the Hubble Space Telescope ( HST ) at a wavelength of \sim 2255 Å , we have directly detected the faint companion at a separation of 0 \farcs 17 .
A second HST observation 1.04 yr later confirms orbital motion in a retrograde direction .
By combining our two measures with the spectroscopic orbit of Kamper and an analysis of the Hipparcos and FK5 proper motions by Wielen et al. , we find a mass for Polaris Aa of 4.5 ^ { +2.2 } _ { -1.4 } M _ { \odot } —the first purely dynamical mass determined for any Cepheid .
For the faint companion Polaris Ab we find a dynamical mass of 1.26 ^ { +0.14 } _ { -0.07 } M _ { \odot } , consistent with an inferred spectral type of F6 V and with the flux difference of 5.4 mag observed at 2255 Å .
The magnitude difference at the V band is estimated to be 7.2 mag .
Continued HST observations will significantly reduce the mass errors , which are presently still too large to provide critical constraints on the roles of convective overshoot , mass loss , rotation , and opacities in the evolution of intermediate-mass stars .

Our astrometry , combined with two centuries of archival measurements , also confirms that the well-known , more distant ( 18 ^ { \prime \prime } ) visual companion , Polaris B , has a nearly common proper motion with that of the Aa , Ab pair .
This is consistent with orbital motion in a long-period bound system .
The ultraviolet brightness of Polaris B is in accordance with its known F3 V spectral type if it has the same distance as Polaris Aa , Ab .