diff --git a/.gitmodules b/.gitmodules
index 8eca4d017..d434c3db8 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -17,3 +17,9 @@
 [submodule "addins/embench-iot"]
 	path = addins/embench-iot
 	url = https://github.com/embench/embench-iot
+[submodule "addins/book_flow"]
+	path = addins/book_flow
+	url = https://github.com/stineje/book_flow
+[submodule "addins/sky130_osu_sc_t18"]
+	path = addins/sky130_osu_sc_t18
+	url = https://foss-eda-tools.googlesource.com/skywater-pdk/libs/sky130_osu_sc_t18
diff --git a/addins/book_flow b/addins/book_flow
new file mode 160000
index 000000000..5ef17d725
--- /dev/null
+++ b/addins/book_flow
@@ -0,0 +1 @@
+Subproject commit 5ef17d725e472ef414e5581bf25805236e6a60ca
diff --git a/addins/sky130_osu_sc_t18 b/addins/sky130_osu_sc_t18
new file mode 160000
index 000000000..83f5245e1
--- /dev/null
+++ b/addins/sky130_osu_sc_t18
@@ -0,0 +1 @@
+Subproject commit 83f5245e1a599c628d6c73e76c1774b8ab5cab91