[ESS] Backslash at the end of R help command hangs ESS 5.3.1

[Michael Hoffman]

Using R 2.1.1, ESS 5.3.1, GNU Emacs 21.3.1 on Linux 2.6.12-1.1381_FC3, 
if I type

 > ?sweep\

(or any help command followed by a backslash)

then the system hangs forever.
-- 
Michael Hoffman