diff --git a/include/photo.h b/include/photo.h
new file mode 100644
index 0000000000000000000000000000000000000000..e7dbb90756e28733c9e93565db120c40ec541c2a
--- /dev/null
+++ b/include/photo.h
@@ -0,0 +1,24 @@
+/** @file photo.h
+ *  @brief Auxiliary header file for photo.c.
+ */
+#ifndef _PHOTO_H_
+
+typedef struct photo_term_t photo_term_t;
+
+struct photo_term_t
+{
+  double A;
+  double lambda;
+
+  /* We store the photoionization terms in a linked list. */
+  photo_term_t *next;
+};
+
+/* Photoionization terms. */
+extern photo_term_t *photo_terms;
+
+#define photo_printf_str "{A = %g, lambda = %g}"
+#define photo_printf_args(_T)  (_T)->A, (_T)->lambda
+
+#define _PHOTO_H_
+#endif