subscripts alignment

[Paolo M. Pumilia]

I noticed that subscripts, in math mode, are not aligned at the same level.
In postscript and dvi translation of the expression (math mode) r_a - r_b 
subscripts a and b lay on different lines, b being on the lower one. 

how to fix that ugly rendering?

thank you for your help

Pol
-- 
 Paolo Pumilia
 email: [EMAIL PROTECTED]
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