using "or" in sparql

[Katja Siegemund]

Hi,

I'm having trouble with constructing a query that uses "or". I already 
tried optional, but it does not lead to the expected result. I'd be glad 
if someone could help me. I'd like to query for:

all var bindings that have no author or no source.

I tried:

SELECT ?r WHERE {?r a ro:FR. FILTER NOT EXISTS {?r ro:hasSource ?a} 
OPTIONAL {FILTER NOT EXISTS {?r ro:isAuthoredBy ?a}}}";

But it only computes the first part (source).

If I have 3 items without a source and 2 others with no author, then I 
should get 5 results instead only the first 3. What am I doing wrong?

Thanks in advance,
Katja