st: Heckman specification

[Johanna Avato <johanna.avato@googlemail.com>]

Does anyone know whether it is mandatory to have the same variables in  
both equations in the heckman two-step procedure (plus the iv in the  
selection equation) or is it possible to drop one variable from the  
selection equation only? I have a model where one variable causes  
major collinearity problems in the selection equation.
THanks.
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/