diff --git a/make-includes/build_lecture_rules.make b/make-includes/build_lecture_rules.make
index 174f9eb..e71be5a 100755
--- a/make-includes/build_lecture_rules.make
+++ b/make-includes/build_lecture_rules.make
@@ -40,11 +40,14 @@
 
 ################################################################################
 #
-# Files to be cleaned by the various "clean" targets.
+# Files to be cleaned by the various "clean" targets. Note that we don't
+# "tidy" .aux files because they may be needed by the xr package for inter-
+# document cross references, but won't get regenerated if the final target
+# PDF files exist. They will be caught by by the "clean" target though.
 #
-TIDY_FILES+=*.tmp *.aux *.out *.log *.nav *.toc *.snm *.head *.dvi \
+TIDY_FILES+=*.tmp *.out *.log *.nav *.toc *.snm *.head *.dvi \
 	slides-combined.pdf slides-notes.pdf
-CLEAN_FILES+=*.pdf
+CLEAN_FILES+=*.aux *.pdf
 
 
 ################################################################################