arcgis desktop - ArcPy 10.1 PointGeometry.projectAs()

I'm trying to reproject an X,Y passed in from an Add-in.

So far this works fine:

pointGeometry = arcpy.PointGeometry(arcpy.Point(x,y),srIn,False, False)
projectedPoint = pointGeometry.projectAs(srOut)

Based on projectAs (spatial_reference, {transformation_name})

I should be able to pass in the transformation_name, but I have no idea where a list of constants may be (says it's a string)?

In .NET I would use 'OSGB1936_To_WGS1984_2', for the transformation I am after.