Title Problem exporting Graphic

Hello

I've recently started using R for Mac. Today I had a problem and I  
didn't know how to solve, that's why I'm sending this mail. Here  it is:
  I was exporting my graph to a .pdf file. The graph's title was  
"Temperature (?C)" in the command window and at Quartz window, but  
when I exported it to a .pdf the title became "Temperature (??C)".  
I've tried to export the graphic as a .jpeg or .png but the problem  
remained. I don't know if it's a bug or if it's normal.

Can anyone help me? Or tell me how to solve this problem if there is  
any?

Thank you Very much
Miguel Leal