  A search for a light charged Higgs boson (H$^+$) decaying to a W boson and a
CP-odd Higgs boson (A) in final states with e$\mu\mu$ or $\mu\mu\mu$ is
performed using data from pp collisions at $\sqrt{s}=$ 13 TeV, recorded by the
CMS detector at the LHC and corresponding to an integrated luminosity of 35.9
fb$^{-1}$. In this search, it is assumed that the H$^+$ boson is produced in
decays of top quarks, and the A boson decays to two oppositely charged muons.
The presence of signals for H$^+$ boson masses between 100 and 160 GeV and A
boson masses between 15 and 75 GeV is investigated. No evidence for the
production of the H$^+$ boson is found. Upper limits at 95% confidence level
are obtained on the combined branching fraction for the decay chain t $\to$
bH$^+$ $\to$ bW$^+$A $\to$ bW$^+\mu^+\mu^-$, of 1.9 $\times$ 10$^{-6}$ to 8.6
$\times$ 10$^{-6}$, depending on the masses of the H$^+$ and A bosons. These
are the first limits for these decay modes of the H$^+$ and A bosons.
