diff --git a/docs/demo-script.md b/docs/demo-script.md
index 2fb41c8..ce3d1f5 100644
--- a/docs/demo-script.md
+++ b/docs/demo-script.md
@@ -5,6 +5,32 @@ copy-pasteable walkthrough against a local `make up` stack.
 
 ---
 
+## S-18a — Objecttypen API up in compose (#139)
+
+**Outcome:** the upstream Maykin **Objecttypen API** runs in the stack — own Postgres + redis, config
+seeded like the other CG modules (`objecttypen-init` runs `setup_configuration` from the
+`rr-objecttypen-config` volume: migrate + provision a dev **static API token**), a health-checked
+`objecttypen` web service on host `:8020`. This is the objecttype catalogue the register record
+(S-18b/S-18c, S-19) will use.
+
+```bash
+make up
+# 1. The API is up; the seeded token authenticates (401 without, 200 with):
+curl -s -o /dev/null -w "%{http_code}\n" http://localhost:8020/api/v2/objecttypes                                   # 401
+curl -s -o /dev/null -w "%{http_code}\n" -H "Authorization: Token 0123456789abcdef0123456789abcdef01234567" \
+  http://localhost:8020/api/v2/objecttypes                                                                          # 200
+#
+# 2. Automated (a CI verify-stack step): asserts both, against the running stack.
+make verify-objecttypen        # → OK — no-auth 401, token 200
+```
+
+**The path:** verbatim upstream image (`maykinmedia/objecttypes-api`, pinned) + the same seed pattern
+as OpenZaak/NRC — `infra/seed-config.sh objecttypen` streams `data.yaml` into an external config
+volume, `objecttypen-init` (RUN_SETUP_CONFIG) applies it. Objecten (S-18b) and the RegisterRecord
+objecttype (S-18c) build on this.
+
+---
+
 ## S-15b — Beheer-portal: default-fill configuration editor (#131, ADR-0026)
 
 **Outcome:** a beheerder edits the ACL's ZGW **default-fill** values (bronorganisatie,