On combined frequency oscillations of the forced Van der Pol oscillator

New results pertaining to combined frequency oscillations of the forced Van der Pol oscillator are derived. Conditions are given on the degree of detuning for which the results are valid. The results obtained are related to those obtained by Adler and Cartwright. Adler's equation is generalized and Cartwright's qualitative result relating to "line drifts" is quantified.