Replace filename with variable in "Activecell.FormulaR1C1=" expression

I am new to Excel VBA so excuse me if I am asking a silly question. I have recorded a macro which contains several lines of code like this one:

ActiveCell.FormulaR1C1 = "='[OCR 14-02-09.xls]sun (2)'!R38C4:R38C5"

I would like to replace the filename "OCR 14-02-09.xls" with a variable. I have tried several different combos of brackets and quotes but always get an error message. Is this possible?

Thanks
pommyb