  Most all-sky searches for continuous gravitational waves assume the source to
be isolated. In this paper, we allow for an unknown companion object in a
long-period orbit and opportunistically use previous results from an all-sky
search for isolated sources to constrain the continuous gravitational wave
amplitude over a large and unexplored range of binary orbital parameters
without explicitly performing a dedicated search for binary systems. The
resulting limits are significantly more constraining than any existing upper
limit for unknown binary systems, albeit the latter apply to different orbital
parameter ranges that are computationally much costlier to explore.
