diff --git a/.gitignore b/.gitignore
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--- a/.gitignore
+++ b/.gitignore
@@ -55,3 +55,6 @@ docs/_build/
 
 # PyBuilder
 target/
+
+# Editors
+.vscode
diff --git a/slep013/proposal.rst b/slep013/proposal.rst
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@@ -0,0 +1,71 @@
+.. _slep_013:
+
+======================================
+SLEP013: ``n_features_out_`` attribute
+======================================
+
+:Author: Adrin Jalali
+:Status: Under Review
+:Type: Standards Track
+:Created: 2020-02-12
+
+Abstract
+########
+
+This SLEP proposes the introduction of a public ``n_features_out_`` attribute
+for most transformers (where relevant).
+
+Motivation
+##########
+
+Knowing the number of features that a transformer outputs is useful for
+inspection purposes. This is in conjunction with `*SLEP010: ``n_features_in_``*
+<https://scikit-learn-enhancement-proposals.readthedocs.io/en/latest/slep010/proposal.html>`_.
+
+Solution
+########
+
+The proposed solution is for the ``n_features_out_`` attribute to be set once a
+call to ``fit`` is done. In many cases the value of ``n_features_out_`` is the
+same as some other attribute stored in the transformer, *e.g.*
+``n_components_``, and in these cases a ``Mixin`` such as a ``ComponentsMixin``
+can delegate ``n_features_out_`` to those attributes.
+
+Testing
+-------
+
+A test to the common tests is added to ensure the presence of the attribute or
+property after calling ``fit``.
+
+Considerations
+##############
+
+The main consideration is that the addition of the common test means that
+existing estimators in downstream libraries will not pass our test suite,
+unless the estimators also have the ``n_features_out_`` attribute.
+
+The newly introduced checks will only raise a warning instead of an exception
+for the next 2 releases, so this will give more time for downstream packages
+to adjust.
+
+There are other minor considerations:
+
+- In some meta-estimators, this is delegated to the
+  sub-estimator(s). The ``n_features_out_`` attribute of the meta-estimator is
+  thus explicitly set to that of the sub-estimator, either via a ``@property``,
+  or directly in ``fit()``.
+- Some transformers such as ``FunctionTransformer`` may not know the number
+  of output features since arbitrary arrays can be passed to `transform`. In
+  such cases ``n_features_out_`` is set to ``None``.
+
+Copyright
+---------
+
+This document has been placed in the public domain. [1]_
+
+References and Footnotes
+------------------------
+
+.. [1] _Open Publication License: https://www.opencontent.org/openpub/
+
+
diff --git a/under_review.rst b/under_review.rst
index ff52d4e..44cfc56 100644
--- a/under_review.rst
+++ b/under_review.rst
@@ -10,3 +10,4 @@ SLEPs under review
 
     slep007/proposal
     slep012/proposal
+    slep013/proposal