A search for a light charged Higgs boson (
H
+
) decaying to a
W
boson and a
C
P
-odd Higgs boson (
A
) in final states with
e
μ
μ
or
μ
μ
μ
is performed using data from
p
p
collisions at
√
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
→
b
H
+
→
b
W
+
A
→
b
W
+
μ
+
μ
−
, of
1.9
×
10
−
6
to
8.6
×
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.