In this article, a new UML extension for the specification of hybrid
systems, where observables may consist of both discrete and
time-continuous parameters, is presented. The construction uses the profile
mechanism of UML 2.0 which is the standard procedure for extending the
Unified Modeling Language. Whereas hybrid modeling constructs are not
available in standard UML, several specification formalisms for this type of
system have been elaborated and discussed in the formal methods communities.
As a basis for the new UML profile, the authors adopt one of these
formalisms - the CHARON language of Alur et.al.- already
possessing several attractive features for modeling embedded real-time
systems with hybrid characteristics. As a result, the profile inherits the
formal CHARON semantics, so it offers the possibility for formal reasoning
about hybrid UML specifications. Conversely, CHARON is associated with a
new syntactic representation within the UML 2.0 world, allowing to develop
hybrid specifications with arbitrary CASE tools supporting UML 2.0 and its
profiling mechanism. The ``look-and-feel'' of the profile is illustrated by
means of a case study of an embedded system controlling the cabin
illumination in an aircraft. The benefits and weaknesses of the constructed
hybrid UML profile are discussed, resulting in feed-back for the improvement
of both UML 2.0 and the CHARON formalism.