  Jets and outflows are ubiquitous phenomena in astrophysics, found in our
Galaxy in diverse environments, from the formation of stars to late-type
stellar objects. We present observations conducted with the 305m Arecibo
Telescope of the pre-planetary nebula CRL 618 (Westbrook Nebula) - a well
studied late-type star that has developed bipolar jets. The observations
resulted in the first detection of 4765 MHz OH in a late-type stellar object.
The line was narrow (FWHM ~ 0.6 km/s) and ~40 km/s blueshifted with respect to
the systemic velocity, which suggests association with the expanding
jets/bullets in CRL 618. We also report non-detection at Arecibo of any other
OH transition between 1 and 9 GHz. The non-detections were obtained during the
observations in 2008, when the 4765 MHz OH line was first discovered, and also
in 2015 when the 4765 MHz OH line was not detected. Our data indicate that the
4765 MHz OH line was a variable maser. Modeling of the 4765 MHz OH detection
and non-detection of the other transitions is consistent with the physical
conditions expected in CRL 618. The 4765 MHz OH maser could originate from
dissociation of H2O by shocks after sublimation of icy objects in this dying
carbon-rich stellar system, although other alternatives such as OH in an
oxygen-rich circumstellar region associated with a binary companion are also
possible.
