  Radial velocity measurements of stellar reflex motion have revealed many
extrasolar planets, but gaps in the observations produce aliases, spurious
frequencies that are frequently confused with the planets' orbital frequencies.
In the case of Gl 581 d, the distinction between an alias and the true
frequency was the distinction between a frozen, dead planet and a planet
possibly hospitable to life (Udry et al. 2007; Mayor et al. 2009). To improve
the characterization of planetary systems, we describe how aliases originate
and present a new approach for distinguishing between orbital frequencies and
their aliases. Our approach harnesses features in the spectral window function
to compare the amplitude and phase of predicted aliases with peaks present in
the data. We apply it to confirm prior alias distinctions for the planets GJ
876 d and HD 75898 b. We find that the true periods of Gl 581 d and HD 73526
b/c remain ambiguous. We revise the periods of HD 156668 b and 55 Cnc e, which
were afflicted by daily aliases. For HD 156668 b, the correct period is 1.2699
days and minimum mass is (3.1 +/- 0.4) Earth masses. For 55 Cnc e, the correct
period is 0.7365 days -- the shortest of any known planet -- and minimum mass
is (8.3 +/- 0.3) Earth masses. This revision produces a significantly improved
5-planet Keplerian fit for 55 Cnc, and a self-consistent dynamical fit
describes the data just as well. As radial velocity techniques push to
ever-smaller planets, often found in systems of multiple planets,
distinguishing true periods from aliases will become increasingly important.
